Inquiry into the medium of instruction (MOI) in Hong Kong has traditionally centered on years 7–9; thus, little is known about school policies and classroom practices in the key senior-secondary years which fall beyond the scope of government diktats. This lacuna is especially marked in the case of Chinese-medium schools, whose students are allowed to take public examinations in English in year 12. This paper tries to illuminate this “grey area” by exploring findings from a large-scale, mixed-methods study which sought to define the nature of MOI policies and practices in Chinese as a medium of instruction (CMI) and English as a medium of instruction (EMI) schools in years 10–12 and the effects of students’ differing school experiences on their English ability and ease of adjustment to English-medium higher education. The findings point out that students preparing for English-based examinations in CMI schools are taught mainly in Cantonese or in a mixture of English and Cantonese, whereas their counterparts in EMI schools are instructed mainly in English. Students from CMI schools reported lower levels of confidence in their English ability, received inferior examination grades and adjusted less well to university study than graduates of EMI schools, thereby highlighting the disadvantages experienced by students assigned to CMI schools on the basis of their primary-school attainments.