Mixture of Spanish and English, whether in isolated loan words or in codeswitching of clauses and sentences, while socially motivated, is subject to clear linguistic constraints. Quantitative analysis of mixing in conversations of MexicanAmericans 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.*