The aim of this study was to investigate vocabulary knowledge and growth across two different language-learning programmes in Hong Kong. The two programmes compared were English immersion programmes (IM) and regular English second-language programmes (RL2). While previous research has identified an overall advantage to IM with respect to language development, comparatively little research on vocabulary development in IM has examined the potential interaction between different types of words (in terms of frequency levels) and different types of vocabulary (passive versus active). Furthermore, very little work has compared these two specific educational contexts in Hong Kong with respect to vocabulary growth. Therefore, three different aspects of vocabulary were measured: passive, controlled active and free active word knowledge at different word-frequency levels in grade 7 and grade 9 students in both IM and RL2. The Vocabulary Levels Tests measured students' knowledge of passive and controlled active vocabulary, whereas students' writing was analysed with the Lexical Frequency Profiles to estimate their free active vocabulary knowledge. Overall, IM students outperform their counterparts in RL2 concerning their knowledge of different types of vocabulary at various frequency levels. IM students also experience a more rapid growth in their vocabulary knowledge, especially for the most frequent 2000 words and academic vocabulary. Such findings support the claim that IM can provide a more favourable context for L2 vocabulary learning than regular L2. The results of the present study are discussed in terms of wider implications for vocabulary learning and the effectiveness of the IM programmes implemented in Hong Kong. (Contains 14 tables and 6 notes.)