This study examines language mixing in 26 Spanish-English dual language learners over the course of their first year of preschool. The children's patterns of language choice while interacting in monolingual language contexts were analyzed at age 3;6 and 4;5 to examine: (1) whether the frequency of language mixing changed during the year; (2) whether mixing was related to proficiency as measured by utterance length and lexical diversity; and (3) whether there were different subgroups of children, among the participants, with similar proficiency and language use patterns. The results indicate that language mixing, which was low at both ages, was related to limited lexical resources only at 3;6. However, by age 4;5, language choice was more constrained by sociolinguistic variables -- children's awareness of the language prescribed by the majority culture -- than by proficiency. An exploratory cluster analysis further reveals different profiles of learners sharing similar proficiency and language mixing characteristics.