Since 1997, the Hong Kong government adopted a ‘biliterate and trilingual’ language policy, aimed at developing biliterate (written Chinese and English) and trilingual (Cantonese, Putonghua and English) citizens. However, Hong Kong secondary schools did not have an agreed approach for this policy implementation, and the Medium of Instruction (MOI) arrangements have been controversial. In response, the government decided to fine-tune MOI arrangements for secondary schools in 2009, allowing them the flexibility to decide on appropriate MOI arrangements. This paper reports on a case study of the trilingual education approach of a Hong Kong secondary school after the introduction of the fine-tuning policy. Research methods employed include questionnaire surveys, interviews and analysis of recorded lessons. Research findings suggest that various factors need to be considered when adopting a trilingual education approach. First, students found it more appropriate to use Cantonese than Putonghua as the MOI for teaching Chinese Language, and the effectiveness of using Putonghua as the MOI (PMI) was questioned by Chinese Language subject teachers and the principal. Second, although some subjects such as Integrated Science and Mathematics were taught using English as the MOI, mixed code is prevalent in classroom teaching. Third, the importance of using the mother tongue in teaching is emphasised. It is hoped that this study will shed light on how trilingual education can be implemented effectively in secondary schools. Copyright © 2020 University of Westminster Press.