Company: AILIM
Filing Date: 2025-09-16
Form Type: 424B2
Source: 0001104659-25-090464
Chunk: 1

Company: Ameren Illinois Co
Filing Date: 2025-09-16
Form: 424B2
Chunk 1
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-1of this prospectus supplement. Neither the Securities and Exchange Commission nor any other regulatory body has approved or disapproved of these securities or passed upon the accuracy or adequacy of this prospectus supplement or the accompanying prospectus. Any representation to the contrary is a criminal offense.

| ​        
 Per bond | ​ 
 ​ | ​ 
 ​ | OfferingPrice(1) 
 ​                | ​ |    103.196% | ​ | ​ 
 ​ | ​ 
 ​ | UnderwritingDiscount 
 ​                    | ​ |    0.875% | ​ | ​ 
 ​ | ​ 
 ​ | Proceeds, BeforeExpenses, toAmeren Illinois 
 ​                                           | ​ |    102.321% | ​ | ​ 
 ​ |
|:---------|:--|:--|:-----------------|:--|------------:|:--|:--|:--|:---------------------|:--|----------:|:--|:--|:--|:--------------------------------------------|:--|------------:|:--|:--|
| Total    | ​ | ​ | ​                | $ | 361,186,000 | ​ | ​ | ​ | ​                    | $ | 3,062,500 | ​ | ​ | ​ | ​                                           | $ | 358,123,500 | ​ | ​ |

(1) In addition to the offering price set forth above, each purchaser will pay an amount equal to the interest accrued on the bonds from September 1, 2025 to the date they are delivered to that purchaser (which amount will aggregate $1,367,187.50 on September 26, 2025). The underwriters expect to deliver the bonds in book-entry form only through the facilities of The Depository Trust Company for the accounts of its participants, including Clearstream Banking, société anonyme , and Euroclear Bank SA/NV, as operator of the Euroclear System, against payment in New York, New York on or about September 26, 2025. Joint Book-Running Managers

| ​ | KeyBanc Capital Markets | ​ | ​ | TD Securities          | ​ | ​ | US Bancorp | ​ |
| ​ | ​                       | ​ | ​ | Fifth Third Securities | ​ | ​ | ​          | ​ |

Co-Managers

| ​ | Academy Securities | ​ | ​ | Cabrera Capital Markets LLC | ​ | ​ | Ramirez & Co., Inc. | ​ |

September 15,