Company: AILIM
Filing Date: 2025-09-16
Form Type: 424B2
Source: 0001104659-25-090464
Chunk: 36

Company: Ameren Illinois Co
Filing Date: 2025-09-16
Form: 424B2
Chunk 36
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 offerings and in other information that we file with the Securities and Exchange Commission. See “Risk Factors” on page 2.

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. Any representation to the contrary is a criminal offense.

These securities may be offered directly or through underwriters, agents or dealers. The terms of the plan of distribution will be provided in the applicable prospectus supplement. See “Plan of Distribution.”

#### The date of this prospectus is October 13, 2023.
TABLE OF CONTENTS

TABLE OF CONTENTS

| ​                                                          | ​ | ​ | Page |   |    |   | ​ |
| About this Prospectus                                      | ​ | ​ | ​    | ​ |  1 | ​ | ​ |
| Ameren Illinois Company                                    | ​ | ​ | ​    | ​ |  1 | ​ | ​ |
| Risk Factors                                               | ​ | ​ | ​    | ​ |  2 | ​ | ​ |
| Where You Can Find More Information                        | ​ | ​ | ​    | ​ |  3 | ​ | ​ |
| Use of Proceeds                                            | ​ | ​ | ​    | ​ |  4 | ​ | ​ |
| Description of Senior Secured Debt Securities              | ​ | ​ | ​    | ​ |  5 | ​ | ​ |
| Description of First Mortgage Bonds and Mortgage Indenture | ​ | ​ | ​    | ​ | 17 | ​ | ​ |
| Description of Senior Unsecured Debt Securities            | ​ | ​ | ​    | ​ | 32 | ​ | ​ |
| Description of Preferred Stock                             | ​ | ​ | ​    | ​ | 39 | ​ | ​ |
| Book-Entry System                                          | ​ | ​ | ​    | ​ | 43 | ​ | ​ |
| Selling Securityholders                                    | ​ | ​ | ​    | ​ | 45 | ​ | ​ |
| Plan of Distribution                                       | ​ | ​ | ​    | ​ | 46 | ​ | ​ |
| Legal Matters                                              | ​ | ​ | ​    | ​ | 47 | ​ | ​ |
| Experts                                                    | ​ | ​ | ​    | ​ | 47 | ​ | ​ |

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TABLE OF CONTENTS

### ABOUT THIS PROSPECTUS
This prospectus is part of a registration statement that we have filed with the Securities