This phenomenological study explores students' decision-making about whether to remain in an optional French immersion (FI) mathematics course in Grade 11, as well as students' subsequent experiences in their mathematics course of choice. Interview data were collected from 10 students who remained in FI mathematics and from six students who did not. The data suggest that for both groups, the decision involves the perceived importance of the FI program, worry and anxiety, and input from parents and others. For students who continued in FI mathematics, reported experiences in the course focused on learning mathematics in the L2, speaking French, use of the L1, and general difficulties. For those who pursued mathematics in English, the reported experiences centred on the transition period and the differences between mathematics in the L1 and the L2.