This study investigates learners' performance in a Taiwanese tertiary content and language integrated learning (CLIL) programme. Learners' English proficiency was measured immediately after entering the programme and before their industrial placement, i.e. after two years. As in previously reported cases, the learners showed a significant improvement in their receptive linguistic skills. They also performed better than other university students in a national-scale English proficiency test. The findings identify a positive correlation between receptive competence and productive competence in English. The correlation between language competence and content achievement was also examined, with the results exhibiting that entry-level language proficiency can affect learners' initial content achievements, but that this effect is not sustained after the first semester as students' language proficiency continued to improve. In the questionnaire survey, the respondents generally agreed with the claimed benefits of the CLIL approach, but were doubtful about the improvement in their productive linguistic skills, enhanced learning motivation and the use of English for instructing content courses. These doubts were most pronounced among those students with lower level language skills or content knowledge. Interviews with CLIL practitioners were also conducted to provide understanding of how they assess CLIL learners' linguistic and content performance. Implications for CLIL practices in Asian contexts are discussed.