Exhibit 31.2

CERTIFICATIONS

I, J. Roe Buckley certify that:

1. I have reviewed this quarterly report on Form 10-Q of Mewbourne Energy Partners 03-A, L.P.

2. Based on my knowledge,
this quarterly report does not contain any untrue statement of a material fact or omit to state a material fact necessary to make the statements made, in light of the circumstances under which such statements were made, not misleading with respect
to the period covered by this quarterly report;

3. Based on my knowledge, the financial statements, and other financial information included in this
quarterly report, fairly present in all material respects the financial condition, results of operations and cash flows of the registrant as of, and for, the periods presented in this quarterly report;

4. The registrants other certifying officers and I are responsible for establishing and maintaining disclosure controls and procedures (as defined in Exchange Act
Rules 13a-15(e) and 15d-15(e)) for the registrant and have:

(a) Designed such disclosure controls and procedures, or caused such
disclosure controls and procedures to be designed under our supervision, to ensure that material information relating to the registrant, including its consolidated subsidiaries, is made known to us by others within those entities, particularly
during the period in which this report is being prepared;

(b) Designed such internal control over financial reporting, or caused such
internal control over financial reporting to be designed under our supervision, to provide reasonable assurance regarding the reliability of financial reporting and the preparation of financial statements for external purposes in accordance with
generally accepted accounting principles;

(c) Evaluated the effectiveness of the registrants disclosure controls and procedures
and presented in this report our conclusions about the effectiveness of the disclosure controls and procedures, as of the end of the period covered by this report based on such evaluation; and

(d) Disclosed in this report any change in the registrants internal control over financial reporting that occurred during the registrants
most recent fiscal quarter (the registrants fourth fiscal quarter in the case of an annual report) that has materially affected, or is reasonably likely to materially affect, the registrants internal control over financial reporting; and

5. The registrants other certifying officers and I have disclosed, based on our most recent evaluation, to the registrants auditors and the
audit committee of the registrants board of directors (or persons performing the equivalent function):

(a) all significant
deficiencies in the design or operation of internal controls which could adversely affect the registrants ability to record, process, summarize and report financial data and have identified for the registrants auditors any material
weaknesses in internal controls; and

(b) any fraud, whether or not material, that involves management or other employees who have a
significant role in the registrants internal control over financial reporting.

Date: August 14, 2009



/s/ J. Roe BuckleyJ. Roe Buckley Executive Vice President Chief Financial Officer Mewbourne Development Corporation, Managing General Partner of the Registrant