abstract in a protocol analysis of second language writing from 20 adult english as a foreign language (efl) iranian students, this research observed how language-switching (l-s), i.e., first language use in l2 writing, was affected by l2 proficiency. switching interactively between first (l1) and second (l2) languages has been recognized as one of the salient characteristics of l2 writing. however, it is not clear how switching between languages is related to l2 proficiency or how switching to the l1 assists writers with differing l2 proficiency in their composing processes. the present study investigated these issues with twenty adult persian-speaking english as foreign language (efl) learners with two levels of proficiency in performing a writing task: an informal personal letter. data were the students think-aloud protocols and written compositions. quantitative and qualitative analyses of these data show that the participants frequencies of language-switching varied slightly by their l2 proficiency, suggesting that l2 proficiency might determine writers’ approaches and qualities of thinking while composing in their l2.