Against that backdrop in comparison for the third quarter of 2021, we were able to post $1.85 and earnings per diluted share, versus $1.76 of adjusted diluted earnings per share in the year ago period.
We grew revenues to $2.52 billion, with 14.5% overall revenue growth and 12.2% organic revenue growth.
We posted 5.4% operating income margins despite strong headwinds from supply chain issues and labor disruptions caused by the Delta Variant.
We grew remaining performance obligations or RPOs 18.7% from the year ago period to $5.38 billion.
We generated operating cash flow of $121 million, despite the strong organic revenue growth.
However, we have seen cost increases of 10% to 20% and anticipate that such increases will continue in the near future.
We still post a decent operating income marked as a 5% against the year ago period of 6.9%.
Further, this segment also bears the brunt of the dollar per gallon increase in the fuel, which gasoline and diesel year-over-year due to its large fleet and this had an impact of 20 to 30 basis points on operating income margins.
Consolidated revenues of $2.52 billion are up $320 million or 14.5% over Quarter 3, 2020 and represent a new all-time quarterly revenue record for EMCOR.
Excluding $50.3 million of revenues attributable to businesses acquired, pertaining to the time that such businesses are not owned by EMCOR and last year's quarter, revenues for the third quarter of 2021 increased nearly $270 million or a strong 12.2% when compared to the third quarter of 2020, which was still somewhat impacted by the effects of the COVID-19 pandemic.
The specifics of each reportable segment are as follows: United States Electrical Construction revenues of $527.9 million increased $55.9 million or 11.8% from 2020s third quarter.
Excluding acquisition revenues within the segment of $29.5 million this segment's revenues grew organically 5.6% quarter-over-quarter.
United States mechanical construction segment revenues of $999.6 million increased $108.1 million or 12.1% from Quarter 3, 2020.
Third quarter revenues from EMCOR's combined United States construction business of $1.53 billion increased $164 million or 12%, with 9.9% of such revenue growth being organic.
United States Building Services Quarterly revenues of $632.5 million increased $75.9 million or 13.6%.
Excluding acquisition revenues of $20.8 million the segment's revenues increased at 9.9% organically.
EMCOR's industrial services segment revenues of $232.2 million increased $60.7 million or 35.4% due to improve demand for both field and shop services, as we are beginning to see some resumption of maintenance and small capital spending in the energy sector.
United Kingdom Building Services revenues of $129.5 million increased $19.4 million or 17.6% from last year's quarter.
Additionally, the segment's revenues were positively impacted by $8 million, a favorable foreign exchange rate movements within the quarter.
Selling, General, Administrative expenses of $243.9 million represent 9.7% of third quarter revenues and compared to $226.8 million or 10.3% of revenues in the year ago period.
The current year's quarter includes approximately $5.3 million of incremental expenses from businesses acquired, inclusive of intangible asset amortization, resulting in an organic quarter-over-quarter increase in SG&A of $11.9 million.
Reported operating income for the quarter of $137.4 million or 5.4% of revenues, compares to operating income of $135.9 million or 6.2% of revenues in 2020's third quarter.
The 80 basis point reduction in operating margin loss due to reductions in gross profit margin within several reportable segments due to a less favorable revenue mix, which I will elaborate on during my individual segment commentary.
Despite this reduction in quarter-over-quarter operating margin, EMCOR's $137.4 million of operating income represent a new third quarter record.
Specific quarterly performance by segment is as follows: Our U.S. Electrical Construction segment operating income of $44.1 million decreased $1.9 million from the comparable 2020 period.
Reported operating margin of 8.3% represents a reduction from the 9.7% margin reported in 2020's third quarter.
In addition, and as disclosed in last year's third quarter, the results from the prior year period benefited from the settlement of final contract value on two projects, which favorably impacted this segments Q3, 2020 operating income and operating margin by $4.4 million and 70 basis points respectfully.
Third quarter operating income for U.S. Mechanical Construction Services segment of $82.3 million represents a $2.3 million increase from last year's quarter, while operating margin of 8.2% represents an 80 basis point reduction from the 9% earned in 2020's 3rd quarter.
Our combined U.S. Construction business is reporting $126.4 million of operating income with an 8.3% operating margin.
Operating income for U.S. building services is $31.6 million or 5% of revenues.
This represents a reduction of $6.9 million and 190 basis points of operating margin quarter-over-quarter.
Our U.S. Industrial Services segment operating loss of $3 million represents a $5.9 million improvement from the $8.9 million loss reported in 2020's 3rd quarter.
U.K. Building Services operating income of $6.6 million or 5.1% of revenues represents an increase of $1.3 million and a 30 basis point improvement and operating margin quarter-over-quarter.
Approximately $400,000 of this period-over-period improvement is due to positive foreign exchange movement, with the remainder attributable to an increase in project activity primarily within the commercial market sector.
Additional financial items of significance for the quarter not addressed in the previous slides are as follows: Quarter three gross profit of $381.3 million is higher than the comparable quarter by $18.2 million or 5%, gross margin of 15.1% as lower than the 16.5% and last year's third quarter due to the shift in revenue mix in each of our U.S. Electrical and Mechanical Construction segments as well as their U.S. building services segment as I just referenced during my segment operating income discussion.
Diluted earnings per common share of $1.85 represents a new quarterly record for the company and compares to $1.11 per diluted share in last year's third quarter.
Non-GAAP diluted earnings per share for the quarter ended September 30, 2020 was $1.76 when compared to our current quarter's performance, we are reporting a $0.09 or 5.1% quarter-over-quarter earnings per share improvement.
Revenues of $7.26 billion represent an increase of $747.8 million, or 11.5%, of which 9.4% of such revenue growth was generated by organic activities.
Operating income of $387.8 million or 5.3% of revenues represents a significant increase from reported operating income for the first nine months of 2020 and a double digit increase from the corresponding adjusted non-GAAP operating income figure for that period.
Year-to-date diluted earnings per share is $5.17 and represents an increase of approximately 14% over 2020's adjusted non-GAAP earnings per share for the nine month period.
For the first nine months of 2021, we have generated approximately $114 million of operating cash flow, which is well below 2020s record performance.
Further, it is important to note that last year's nine month operating cash flow was favorably impacted by $82.3 million due to government stimulus measures that allow for the deferral of certain tax payments in both the United States and the United Kingdom.
As previously communicated, my expectation for full year 2021 was operating cash flow in excess of $300 million.
With our upward revision in 2021 revenue expectations, I am still targeting the same level of operating cash flow performance, but it is possible that we may not eclipse the $300 million target should our working capital investment be greater than expected during the fourth quarter.
Cash on hand is down from year-end 2020 driven by cash used in financing activities are approximately $213 million, inclusive of $183 million used for the repurchase of our common stock and cash used in investing activities of $137.5 million, most notably due to payments for acquisitions that have cash acquired totaling approximately $114 million.
These uses of cash were partially offset by cash provided by operations of $114 million as I noted just a few moments ago.
Working capital has increased by nearly $20 million.
Net identifiable intangible assets increased by $19 million as the impact of additional intangible assets recognize the connection with the previously referenced acquisitions which was largely offset by $48 million of amortization expense during the year-to-date period.
As a reference point, on a full year basis we anticipate depreciation and amortization expense, including both depreciation of property, plant and equipment, as well as amortization of intangible assets to be approximately $112 million for 2021.
And EMCOR's debt-to-capitalization ratio has reduced to 11.4% from 11.9% at year-end, 2020.
And I'm going to be on page 12 remaining performance obligations by segment and market sector.
As mentioned earlier, total company RPOs at the end of the third quarter were just under $5.4 billion, up $849 million or 18.7% when compared to the year ago level of $4.5 billion.
Organic RPO growth was strong 15.6%.
Year-to-date for the nine months completed in 2021 total RPOs have increased $784 million, or just over 17%.
Our two domestic construction segments experienced strong construction project growth in the quarter, with RPOs increasing $606 million or 16.5% from the same period last year.
Building Services or RPO levels increased 180 million or almost 29% from the year ago quarter, 142 million and 180 million was organic.
Our Industrial Services segments, our RPO increase of $53 million from September 2020.
I'm now going to finish our discussion on pages 15 and 16.
Our new guidance is diluted earnings per share of $6.95 to $7.15 and we now expect our revenues between $9.80 billion and $9.85 billion.
To date into 2021, we have repurchased $183 million in EMCOR stock, paid $21 million in dividends and invested $114 million in acquisitions that will continue to position EMCOR for long term and sustained growth.
Our board of directors just authorized a new and our largest share repurchase authorization of an additional $300 million.

Summary:
Against that backdrop in comparison for the third quarter of 2021, we were able to post $1.85 and earnings per diluted share, versus $1.76 of adjusted diluted earnings per share in the year ago period.
We grew revenues to $2.52 billion, with 14.5% overall revenue growth and 12.2% organic revenue growth.
Consolidated revenues of $2.52 billion are up $320 million or 14.5% over Quarter 3, 2020 and represent a new all-time quarterly revenue record for EMCOR.
Diluted earnings per common share of $1.85 represents a new quarterly record for the company and compares to $1.11 per diluted share in last year's third quarter.
As previously communicated, my expectation for full year 2021 was operating cash flow in excess of $300 million.
With our upward revision in 2021 revenue expectations, I am still targeting the same level of operating cash flow performance, but it is possible that we may not eclipse the $300 million target should our working capital investment be greater than expected during the fourth quarter.
Our new guidance is diluted earnings per share of $6.95 to $7.15 and we now expect our revenues between $9.80 billion and $9.85 billion.
Our board of directors just authorized a new and our largest share repurchase authorization of an additional $300 million.