This exploratory study focuses on four non-native English speaking secondary content teachers in a short-term immersion program aimed at introducing them to language teaching methods for secondary school content instruction through the medium of English. Such programs have been found to have largely mixed results for language performance. This may be especially true of programs for teachers because task adequacy in teaching may not be shown by improved scores on traditional language performance measures. This study uses multiple complexity, accuracy and fluency measures to analyze transcripts of paired peer microteaching tasks at the beginning and end of a ten-week program. Analysis of the results shows that in general the teachers' language performances improved, but that there were complex interactions among the measures. In general the performance measures reflected task adequacy, but they may have been affected by nonlinguistic factors such as teaching aims. Although the study is limited because of its small size, it suggests that teacher language performances are complex and dynamic, and that even short-term programs with non-linguistic aims can have a positive effect on language performances. (Contains 4 tables.)