Studies on the acquisition of a third language (L3) in a bilingual context have shown that literacy in two languages facilitates the acquisition of a third (Cenoz & Valencia, 1994; Swain, Lapkin, Rowen, & Hart, 1990). The present study seeks to contribute to this line of research by comparing the acquisition of English as an L3 by Catalan/Spanish bilingual high school students in an immersion program with the acquisition of English by Spanish monolinguals. Data from 201 participants were submitted to a hierarchical multiple regression analysis, rendering results that show that bilingualism indeed has a positive effect on the acquisition of an L3. The evidence is discussed from a cognitive perspective.