The present study examines the development of second language (L2) vocabulary in writing comparing two different learning contexts, formal instruction (FI) at home followed by a 3-month stay abroad (SA). Data were obtained from a group of 30 Catalan/Spanish advanced learners of English before and after each learning period by means of a written composition. These compositions were analyzed quantitatively in terms of lexical measures in various domains, such as diversity, density, sophistication and accuracy. Baseline data from 29 native speakers of English, elicited through the same task, were also used for comparison purposes. Our results reveal that the different aspects of lexical proficiency in L2 writing do not develop in the same manner across the two learning contexts. While SA is particularly beneficial for lexical diversity, lexical accuracy seems to flourish after both FI and SA learning periods. Learners also achieve more native-like behaviour in terms of spelling and lexical sophistication after SA. These findings are interpreted together with the relevance of the selected measures used in the study.