Content-based instruction (CBI) adopts a second language (L2) as the medium of instruction for some or all academic subjects to facilitate L2 learning. There seem however, no uniform policies concerning which academic subjects should be taught in L2, in case only some subjects are involved. Conventional wisdom tends to favour Humanities subjects since they are more verbal and hence more conducive to L2 learning. However, there has been no empirical evidence supporting such claim so far. This study attempts to address such a gap by comparing students' language learning opportunities in different academic subjects. Twenty-two lessons across grades and subjects were observed in two English-medium secondary schools in Hong Kong (where CBI was practised). By estimating the proportion of student talk, mean length of student turns, Initiation-Response-Feedback (I-R-F) sequences and form-focused exchanges, the language learning opportunities in Humanities and Science lessons were compared. It was found that in Humanities lessons, students talked significantly more and longer. There were also longer chains of I-R-F sequences where teachers elicited more student responses and provided feedback on students' L2 output. Such differences have significant implications for L2 learning and bilingual education programmes.