While there is ample evidence that study abroad (SA) enhances oral fluency in a foreign language, the effects of different types of learning context on other aspects of oral skills, such as vocabulary use, have not received much attention in academic research and are less clear. The present study tries to fill this void by investigating lexical development in oral production by advanced learners of English experiencing two different contexts of acquisition: formal instruction (FI) at home, followed by a 3-month stay abroad (SA). Speech samples were elicited from a group of 30 Catalan/Spanish undergraduates before and after each learning context by means of an oral interview and were later analyzed in terms of lexical diversity, sophistication, density and accuracy. Additionally, we examined baseline data from 25 native speakers of English, elicited through the same task. Our results reveal that both contexts enhance significant development in lexical accuracy and that SA proves especially beneficial for growth in adverb density, which moves towards target-like norm and adds fluidity to learners' speech. FI, in contrast, causes a significant impact on lexical sophistication, as learners seem to acquire more specific vocabulary during classroom instruction.