In higher education, understanding discipline-specific vocabulary can be challenging. Insufficient vocabulary can be a major challenge for students as they begin their university studies. This study examined university undergraduate English language learners' discipline-specific vocabulary retention when they used an in-house developed mobile app, Books vs Brains@PolyU, in Hong Kong. A total of 159 second language (L2) students from four disciplines participated in the study, and they completed a pre- and posttest consisting of 120 words in four difficulty levels in their chosen disciplines. The results suggested that participants acquired more vocabulary words at the intermediate and advanced levels than at the beginner and elementary ones. The findings improve the understanding of MALL and vocabulary acquisition using apps to facilitate and support learners' academic studies. Copyright © 2020 The Author(s).