Résumé : In this article we reconstruct the actuation and transmission of a phonological innovation known as prefix reduction within the Kikongo language cluster situated in the wider Lower Congo region of Central Africa. We argue that this change spread from a focal area coinciding with the heartland of the Kongo kingdom as a classical process of dialectal diffusion. Thanks to a unique Kikongo corpus that starts in the 17th century, we can provide diachronic empirical evidence for different phases of the process, which has been otherwise difficult, if not impossible, in Bantu historical linguistics. What is more, and also quite exceptional in African linguistics, we have fairly good insight into the 'social ecology' of this language change and argue that political centralization and economic integration within the realm of the Kongo kingdom facilitated such a contact-induced diffusion between closely-related language varieties.