The present study examines the possible effects of bilinguality on third language learning among English as a Foreign Language (EFL) learners with a focus on reading comprehension proficiency achievement. This study is a longitudinal survey of EFL learners in Urmia University in Azerbaijan, Iran, during 2002-2004. It compared 56 Turkish-Persian bilinguals with their 42 Persian monolingual peers in terms of their performance on the First Certificate of English (FCE) tests of reading comprehension in three phases. The subjects, who fell within the age range of 18-24, were similar in social (socioeconomic status), psychological (motivation and attitudes) and educational (previous exposure to other languages) orientations. Analysis of the data submitted to independent t -tests indicated that bilinguals performed significantly better than monolinguals in the reading comprehension tests in all the three phases of investigation, thus strengthening the argument that bilingualism may be a good predictor of achievement in learning a third language. Further analysis of the language use patterns among bilinguals provided evidence to support the proposition that the effects of bilingualism on additional language learning may be mediated by sociocultural factors. The findings are discussed in relation to the Interdependence Hypothesis, which posits positive interrelationships between students' L1 and their L2 development, if the society provides enough support for both languages. (Contains 4 tables and 1 figure.)