This study sought to examine to what extent language learners in an immersion classroom used their native language and the foreign language when performing the cognitive operations involved in math word problems. Thirty-two third- through sixth-graders in a Spanish immersion program participated in the study. Data were collected through: (1) interviews; (2) student questionnaires that covered pupils' abilities, attitudes, and preferences with regard to thinking in Spanish; (3) classroom observations; and (4) background information such as school grades, achievement test scores, and extracurricular exposure to Spanish. The findings revealed that English seemed at times to play a more prominent role in the students' internal language environment than did Spanish. In responding to word problems in math, fifth- and sixth-graders reported favoring English in their cognitive processing and were also observed to be doing so. They would read the problem in Spanish but would shift to English immediately or as soon as they had some conceptual difficulty. (Contains 18 references.) (MDM)