The question of how migrants’ language use affects their ethnic identity has received considerable attention in the literature. There is, however, little understanding of how this relationship manifests or is negotiated in interethnic families. This article presents an in-depth exploration of Spanish mothers’ experiences of Spanish- and English-language interactions with their English-born children. Semistructured interviews were conducted with Spanish mothers living in Britain in interethnic partnerships and transcripts were subjected to interpretative phenomenological analysis. Analysis reveals a process of identity change where participants’ shifting ethnic identifications with host and heritage culture is intimately related to their language use with their children. Pivotal to this process is the participants’ need to maintain their “Spanish mother” identity, a desire that can only be fulfilled by transferring their heritage language to their children and speaking it with them. Findings reveal how this dynamic affects perception of family roles, relationship quality, and psychological well-being.