Although word-initial consonants are highly salient cross-linguistically, the process of initial-consonant mutation has nonetheless continued to affect them in the Celtic languages. This paper investigates the use of the mixed mutation (MM) in Breton, following the progressive particle. Like all mutation, it naturally affects the phonology, but also gives (redundant) information for morphosyntax. Mutation is generally presumed to be a regular process, but as there has been a gap in the transmission of Breton, the extent to which this phono-syntactic phenomenon is consistent across generations remains open to discussion. It has been claimed that younger speakers, being strongly French-dominant, do not use mutation correctly. We tested this examining both distribution of usage and acoustic measurements of the consonants in question. Data from original fieldwork indicate that young adults use MM in the same way as older speakers, but children attending Breton-medium schooling are less proficient. Mixed mutation is difficult to acquire, the crucial factor being sustained Breton input beyond the early teenage years. Acoustically, there is no difference in the production of MM cross-generationally. The difference between the two generations is in the use of the progressive particle itself, omitted by the older generation, but retained by younger speakers.