This paper analyzes patterns of code switching found in a small group of first and second generation Catalan immigrants in Mexico, and is part of a wider study on the impact of exile upon the construction of identity in the discourse of political immigrants in Latin America. With regard to the pragmatic exploitation of code switching, it argues that code switching is used by bilinguals even when a monolingual alternative is available, either because changing languages at a specific point in conversation reduces the processing effort involved in interpreting the stretch of discourse where the switch occurs, or because the shift of code adds cognitive effects to the overall interpretation of the message. The paper focuses on the effects of "ad hoc" concept construction,--a well studied phenomenon in monolingual discourse--on the presentation of self in bilingual discourse, analyzing cases when the pragmatically derived "ad hoc" concept involves a switch of codes. It is argued that this mechanism helps subjects represent themselves in bilingual conversation as distinct both from Mexicans and from non expatriate Catalans.