In September 1998, the Education Department of the fledgling Hong Kong Special Administrative Region implemented a controversial medium-of-instruction policy which compelled around three-quarters of the territory's hitherto English-medium secondary schools to switch to Chinese-medium teaching in Forms 1-3 (years 7-9). Only 112 schools were permitted to remain English-medium, but only on the condition that their content-area teachers make consistent use of English as the language of classroom instruction and interaction rather than, as had been the case in the last two decades of colonial rule, a mixture of Cantonese and English. This article presents the findings of a study which sought to determine the extent to which the post-colonial government's language policy has been translated into classroom practice in the reformed Chinese and English streams since 1998. The findings were derived from a questionnaire survey of a sample of undergraduates from the first two cohorts to complete their studies in the reformed Chinese and English streams, together with semi-structured interviews with students and teachers currently studying and teaching in the two streams. The findings indicate that teachers in both the Chinese and English streams have found it difficult to fully implement the new policy in their classrooms. (Contains 6 figures and 6 notes.)