ABSTRACT This study investigates English-medium instruction (EMI) academic motivation at a Chinese transnational university, aiming to bridge identified research gaps by employing the Situated Expectancy-Value Theory (SEVT). Specifically, it explores motivational predictors of EMI academic performance and the factors influencing these predictors. A longitudinal mixed-methods approach was adopted, utilising a validated Chinese EMI Student Academic Motivation Scale administered twice (n = 156) over a semester and semi-structured interviews with a subgroup of 15 participants. Results from multiple linear regression indicated that the combination of nine motivational variables had a moderate impact on EMI academic performance, and two of the predictor variables – expectations of success and emotional cost – were statistically significant predictors. Qualitative findings from the interviews revealed four key factors that influence the development of expectations of success: past learning experience, English language ability, learner autonomy, and socialisers’ influence; additionally, three main factors were identified as affecting the development of emotional cost: English language barriers, EMI curriculum, and peer pressure. The study concludes with practical pedagogical recommendations, drawn from both quantitative and qualitative data, aimed at enhancing EMI effectiveness in similar educational contexts.