This study explored changes in English language proficiency and several non-linguistic factors during four years of English medium instruction (EMI) in two academic disciplines in a Turkish university setting. Moreover, it also investigated whether changes (if any) had a predictive impact on the academic success of EMI students. In addition, potential differences between disciplines were also investigated. The participants were 241 EMI students from Business Administration (n = 117) and Mechanical Engineering (n = 124) programmes. Our findings revealed that in addition to the language proficiency scores, various non-linguistic factors, including self-efficacy, ideal L2 self, motivation, self-regulation skills, and anxiety levels, changed throughout EMI education. However, only English proficiency and instrumental motivation emerged as positively significant predictors of EMI success. Our findings also revealed that the increase in participants' intrinsic motivation scores was a significant negative predictor of EMI success. These results are discussed and implications are given regarding the impact of linguistic and non-linguistic factors in EMI contexts.