The purpose of this study is to compare the vocabulary learning of monolingual learners of English as a second language with bilingual learners of English as a third language. The study is based on data from 52 monolingual Persian-speaking learners of English and 45 bilingual Azeri-Persian-speaking learners of English. All the participants were females studying English as a foreign language at two universities in Iran. The informants were exposed to two incidental and four intentional vocabulary learning exercises. They were then measured at four difficulty levels using the Vocabulary Knowledge Scale (Paribakht & Wesche, 1997). Variables such as English language proficiency, intelligence, family educational background, gender, age and type of university were controlled. The third language learners outperformed their second language counterparts at all word difficulty levels. The findings are discussed in relation to bilinguals' higher level of executive and inhibitory control. (Contains 2 tables and 2 figures.)