A good command of academic vocabulary is important for academic success in higher education. However, research has primarily focused on the receptive academic vocabulary knowledge of L2 learners while devoting relatively limited attention to their productive use of such vocabulary and its impact on writing quality. To address this gap, we analysed the problem-solution essays written by 168 first-year undergraduates in Hong Kong, focusing on the relationship between their use of academic words in the Academic Vocabulary List (AVL) and the overall quality of their writing. We also explored the relationship between the size of students’ receptive academic vocabulary and the frequency of its use in writing. Findings revealed that essays with high scores contained a greater density and diversity of academic vocabulary than low-scored essays, with greater frequency of words in the 1–500 and 501–1000 tiers of the AVL significantly predicting better writing quality. The essays also showed a significant relationship between the participants’ receptive academic vocabulary size and the diversity of academic words used in writing. However, no significant relationship was observed between receptive academic vocabulary size and the density of academic words used. We highlight the implications of these findings for EAP teaching and research. • Problem-solution essays written by undergraduates in Hong Kong were analysed. • Density and diversity of academic vocabulary (AV) predict L2 writing quality. • Learners’ receptive AV size significantly relates to AV diversity in their writing. • Only words from two tiers of the AVL significantly predicted writing scores. • A holistic and tiered approach to assessing AV use is important.