The purpose of this study is twofold: first, to estimate the contribution of discipline-related knowledge and English-language proficiency to reading comprehension in English for academic purposes (EAP) and, second, to specify the levels at which the compensatory effect between the two variables takes place for successful EAP reading. The participants in the study were 380 native Spanish-speaking undergraduates who exhibited a wide range of proficiency in English as a foreign language and knowledge of the topics being tested. Scores for the 3-criterion variables (discipline-related knowledge, English proficiency level, and academic reading) were subjected to 6 multiple regression analyses. The results indicated that English proficiency accounted for a range varying between 58% and 68% of EAP reading, whereas discipline-related knowledge accounted for a range varying between 21% and 31%. The exact levels at which the compensatory effect between the two variables takes place for successful academic reading are provided, and pedagogical implications are suggested.