English is increasingly common as a medium of instruction in higher education internationally, and evidence of English language proficiency (ELP) forms a vital aspect of many post-secondary admission applications. Scores from different ELP measures (e.g. standardized assessments; institution-specific English programmes) are often used as equivalent assessment of language skills for admission, yet little research exists to show equivalency to one another. This study examines three measures of ELP used for post-secondary admission: scores from IELTS, TOEFL, and an institution-specific academic English programme, relative to course grade results, and student retention rates in a first-year undergraduate engineering programme. Statistically significant differences were found between the three types of ELP assessment with respect to academic success. Furthermore, students admitted using the academic English programme showed a statistically significantly higher rate of first-year programme dropout. These findings call into question the assumption of equivalency among measures of ELP used for post-secondary admission.