This study explored changes in English language proficiency and non-linguistic factors over four years of English Medium Instruction (EMI) in two academic disciplines at a Turkish university. It investigated whether these changes predicted academic success and whether there were differences between disciplines. The participants were 241 EMI students, 117 in Business Administration (social sciences) and 124 in Mechanical Engineering (engineering). Findings revealed that language proficiency and instrumental motivation were significant positive predictors of EMI success, while increased intrinsic motivation was a significant negative predictor. Non-linguistic factors such as self-efficacy, ideal L2 self, motivation, self-regulation, and anxiety also changed over time. However, only language proficiency and instrumental motivation significantly predicted academic success. In social sciences, language proficiency and instrumental motivation were significant predictors, while in engineering, only intrinsic motivation had a negative impact on EMI success. The study highlights the importance of language proficiency and instrumental motivation in EMI success, and suggests that EMI is not a one-size-fits-all approach, with different needs and pathways to success for different disciplines. The findings have implications for educational practice, emphasizing the need for tailored instruction and the importance of instrumental motivation in EMI contexts.This study explored changes in English language proficiency and non-linguistic factors over four years of English Medium Instruction (EMI) in two academic disciplines at a Turkish university. It investigated whether these changes predicted academic success and whether there were differences between disciplines. The participants were 241 EMI students, 117 in Business Administration (social sciences) and 124 in Mechanical Engineering (engineering). Findings revealed that language proficiency and instrumental motivation were significant positive predictors of EMI success, while increased intrinsic motivation was a significant negative predictor. Non-linguistic factors such as self-efficacy, ideal L2 self, motivation, self-regulation, and anxiety also changed over time. However, only language proficiency and instrumental motivation significantly predicted academic success. In social sciences, language proficiency and instrumental motivation were significant predictors, while in engineering, only intrinsic motivation had a negative impact on EMI success. The study highlights the importance of language proficiency and instrumental motivation in EMI success, and suggests that EMI is not a one-size-fits-all approach, with different needs and pathways to success for different disciplines. The findings have implications for educational practice, emphasizing the need for tailored instruction and the importance of instrumental motivation in EMI contexts.