Lexical richness in written language production has been examined in different contexts and in relation to different genres but research into the effects of within-genre topics on lexical richness is still limited and takes little account of learners’ perspectives. The present study therefore sought to understand the effects of topics on lexical richness (lexical density, lexical diversity and lexical sophistication) in written essays by Vietnamese EFL learners. Data were collected from sixty-four English-major university students who each wrote four essays on four different topics across two genres (persuasive and descriptive) in their normal classes as progress tests. Ten students were also interviewed after they had completed all the essays to understand their perceptions of the writing topics. Lextutor’s VocabProfile was used to obtain measures of lexical density, lexical diversity and lexical sophistication. The results showed mixed effects of writing topics on these three lexical measures. Learner perspectives provided insights into how cognitive and affective individual factors contributed to the complexity of topic influence. Pedagogical implications are discussed in regard to how to enhance productive lexical use in writing through task selection and instructional planning.
