Teaching science through English as a medium of instruction (EMI) is a growing phenomenon around the world. In Hong Kong, this was realised on a large scale in 2010, with the implementation of a “fine-tuning” compulsory language policy. This allowed Chinese-medium schools to adopt EMI fully. Yet, despite such rapid and widespread adoption, an adequate understanding of key stakeholders’ experiences in relation to their perceptions of what constitutes effective EMI science education remains scarce. Thus, we question the sustainability of EMI programs that are driven by top-down policy. In this case study, we explore the perspectives and experiences of six EMI science teachers and thirteen of their students as their secondary school transitions from partial to full EMI. From in-depth interviews (complemented by classroom observations), findings reveal that the transition to full EMI has presented challenges that appear to hinder students’ development of scientific knowledge and the language of science in English. This directly counters the primary goal of the fine-tuning policy. Nevertheless, findings also illuminate a number of coping strategies teachers and students use to deal with their changing curricula. Overall, we offer insights into this under-researched context of transitioning EMI programs and provide recommendations for future research and practice.