The present study analyses bilingual conversations taking place in binational families living in the UK. The focus is on two Polish/English stepfamilies, where the Polish mothers' concern with preserving and developing their children's linguistic and cultural identity collides with the English partner's limited knowledge of Polish. While the paper takes a CA approach to bilingual conversation (Auer 1984; Li 1994), it shows that even when language choice is rational and participant- rather than discourse-related, it can have a crucial impact on the discourse identities of the participants in the interaction. The application of Goffman's participation framework (1981) to the analysis of the data makes it possible to investigate different functions of and motivations for language choice and their impact on the participation status of the English participants--as well as their identity.