The aim of this study was to investigate how second language (L2) productive vocabulary develops over time by targeting multiple dimensions of word knowledge. The research questions addressed were: a) How does the productive vocabulary of 11-12 year-old L2 learners in an intensive ESL program in Quebec develop over time with regard to vocabulary size, lexical richness, and lexical depth? and b) What similarities or differences can be observed between written vocabulary development and spoken vocabulary development? The study drew on a 58,000-word written corpus and a 28,000-word spoken corpus produced by 108 beginner-level francophone learners of English (11-12-year-olds). Data analyses were based on several measures which included counts of word families, percentages of 1K, 2K, and 3K+ words, types-per-family ratios, and counts of 2-word lexical bundles, all analyzed using the tools available at www.lextutor.ca. Two-way analyses of variance (ANOVAs) with modality (written, oral) and time (T1, T2) as between-participants factor were used to compare the four learner corpora (written and oral at T1 and T2). Findings indicate that learners’ written vocabulary developed in all three dimensions (vocabulary size, lexical richness, and lexical depth), and that their spoken vocabulary improved only in lexical richness but showed no significant growth in size and lexical depth. The results suggest that overall learners performed better in written than in spoken modality, which could be attributed to a) task-specific effects, b) differences between spoken and written language access, and c) differences in the nature of spoken and written vocabulary used.