This study investigates how the lexical and syntactic characteristics of L2 learners’ academic writing change over the course of a one-month long intensive English for Academic Purposes (EAP) programme at a British university. The participants were asked to produce two argumentative essays, at the beginning and at the end of the EAP course, which were analysed using measures that are theoretically motivated by previous research in corpus linguistics, systemic functional linguistics, and developmental child language acquisition. The results indicate improvements, with regard to lexical diversity, both for intermediate-level students who were preparing for undergraduate university studies in the UK and upper-intermediate level participants who were planning to continue their studies at postgraduate level. The academic argumentative texts of the students in the lower proficiency group also demonstrate development in noun-phrase complexity and in the use of genre-specific syntactic constructions. The findings suggest that despite no explicit focus on lexis and syntax in the EAP programme, by the end of the course the students’ writing exhibited a developmentally more advanced repertoire of lexical and syntactic choices that are characteristic of expository texts in academic contexts.