This study explores the changes in English language proficiency and several non-linguistic factors during four years of English Medium Instruction (EMI) in two academic disciplines—Business Administration and Mechanical Engineering—in a Turkish university. The research investigates whether these changes have a predictive impact on EMI students' academic success and examines potential differences between the two disciplines. The study involved 241 EMI students, with 117 in Business Administration and 124 in Mechanical Engineering. Key findings include: 1. **Linguistic and Non-Linguistic Factors**: Various non-linguistic factors, such as self-efficacy, ideal L2 self, motivation, self-regulation skills, and anxiety levels, changed throughout EMI education. 2. **Predictive Impact**: Only English proficiency and instrumental motivation emerged as positively significant predictors of EMI success. 3. **Intrinsic Motivation**: An increase in intrinsic motivation scores was a significant negative predictor of EMI success. 4. **Disciplinary Differences**: In Business Administration, language proficiency and instrumental motivation significantly predicted EMI success, while in Mechanical Engineering, only language proficiency and instrumental motivation were significant. 5. **Dynamic Nature of Factors**: The study highlights the dynamic nature of these factors, with some improving over time and others declining. The implications for education and research suggest that continuous exposure to English positively impacts language proficiency and that EMI programs should tailor their approaches to meet the specific needs of different disciplines. Additionally, future studies should focus on longitudinal designs to better capture the dynamic nature of these factors.This study explores the changes in English language proficiency and several non-linguistic factors during four years of English Medium Instruction (EMI) in two academic disciplines—Business Administration and Mechanical Engineering—in a Turkish university. The research investigates whether these changes have a predictive impact on EMI students' academic success and examines potential differences between the two disciplines. The study involved 241 EMI students, with 117 in Business Administration and 124 in Mechanical Engineering. Key findings include: 1. **Linguistic and Non-Linguistic Factors**: Various non-linguistic factors, such as self-efficacy, ideal L2 self, motivation, self-regulation skills, and anxiety levels, changed throughout EMI education. 2. **Predictive Impact**: Only English proficiency and instrumental motivation emerged as positively significant predictors of EMI success. 3. **Intrinsic Motivation**: An increase in intrinsic motivation scores was a significant negative predictor of EMI success. 4. **Disciplinary Differences**: In Business Administration, language proficiency and instrumental motivation significantly predicted EMI success, while in Mechanical Engineering, only language proficiency and instrumental motivation were significant. 5. **Dynamic Nature of Factors**: The study highlights the dynamic nature of these factors, with some improving over time and others declining. The implications for education and research suggest that continuous exposure to English positively impacts language proficiency and that EMI programs should tailor their approaches to meet the specific needs of different disciplines. Additionally, future studies should focus on longitudinal designs to better capture the dynamic nature of these factors.