Immediately after the handover of Hong Kong to China, the teaching of Chinese in Putonghua was promulgated by the Hong Kong SAR government as a long-term goal. However, no clear definition and guidance regarding Putonghua instruction has been provided to teachers to facilitate the smooth implementation of this change. A review of the literature indicates that teachers' beliefs have great value in assisting them to make sense of changes and govern their thoughts and behaviours. However, little is known about how teachers' beliefs help teachers to understand the transition to a new medium-of-instruction (MOI). Thus, the aim of this case study is to examine how teachers' beliefs help teachers to conceptualize the Putonghua medium-of-instruction (PMI) and how these beliefs affect their curricular decisions as well as the implementation of PMI. Data were collected from 10 secondary Chinese Language teachers in one school using semi-structured interviews and classroom observations. The findings revealed that since teachers held misconceptions about PMI in the initial phase, the implementation of PMI led to a shift of focus away from Chinese content knowledge. In the second phase, PMI was perceived merely as a code-switching device, and, as a result, only a surface change occurred that appeared unlikely to facilitate effective teaching and deep learning. This paper concludes with the observation that code switching alone is insufficient to achieve a successful transition of MOI if the deeper issue of teachers' beliefs is neglected. (Contains 2 tables and 6 notes.)