This study aims at the written production of students of English as a foreign language in Brazil. Compositions of intermediate and advanced students are analyzed with a special focus on constructions whose inadequacies may be specifically associated to vocabulary usage. The general goal of the research is to investigate the interference of Portuguese in the acquisition of English as a foreign language, with a special emphasis on lexical usage in the written modality. Its specific objective is to test the hypothesis that the non isomorphic character of sense relations between different languages − in the case of this study, Portuguese and English − is related to the incidence of lexical problems encountered in compositions of students of English as a foreign language. The results of the analysis, which was mainly grounded on the propositions of Cruse (1986), Lyons (1990) and Saeed (2003), contribute to confirm this hypothesis, demonstrating that the lexical problems being scrutinized can be associated with mother tongue interference, and, more specifically, with the discrepancies in the sense relations between Portuguese and English.