A 3-year study was launched in a Hong Kong secondary school to investigate the effects of the medium of instruction (MOI), specifically English and Chinese, on the learning of certificate-level physics. A total of 199 Secondary Four (S4 or tenth-grade) students, divided into three major ability groups, participated in a teaching intervention designed to determine the effects of MOI on their learning achievement and motivation. The results of conceptual assessments and physics examinations revealed Chinese to be a superior MOI in enabling low-ability students to attain a higher level of achievement, whereas English was more suitable for their high-achieving counterparts. However, little conclusive evidence regarding the role of MOI for the medium-ability groups was found. A questionnaire-based survey indicated that students were more motivated to learn physics through Chinese as the MOI (CMI) rather than English (EMI), although significant limitations to its use were identified for the topic of "Heat." Deficiencies in the vocabulary needed for abstract scientific concepts in Chinese may account for these limitations (for instance, Chinese uses the same word, "re" ([Chinese character omitted]), for both "heat" and "hot"). Finally, follow-up interviews at the end of the study revealed a sharp contrast between the learning prospects of EMI and CMI students.