Mixture of Spanish and English, whether in isolated loan words or in code-switching of clauses and sentences, while socially motivated, is subject to clear linguistic constraints. Quantitative analysis of mixing in conversations of Mexican-Americans suggests specific functional constraints to express tense/aspect/mood and subject/object relationships, as well as structural constraints which permit only surface structures which are grammatical in both languages. Resolution of structural conflict plays a key role, so that lexical cores trigger longer phrasal switches if they govern rules which create non-shared surface structures. The relative frequency of mixes without structural conflict is constrained by discourse function.