This study investigates the development of lexical and syntactic complexity in argumentative writing among 80 Iranian EFL learners aged 15 to 35, enrolled in an intensive eight-week English for Academic Purposes (EAP) program targeting B2 and C1 proficiency levels. Employing a mixed-methods design, it analyzes pre- and post-intervention essays using automated tools such as Coh-Metrix and VocabProfile alongside qualitative interviews. Significant gains were found in vocabulary richness, collocation usage, and rhetorical device employment, primarily among advanced (C1) learners, evidencing deeper lexical sophistication. Parallel improvements in sentence complexity, subordination, and syntactic variety highlight evolving structural maturity, with B2 learners focusing on clausal elaboration and C1 learners achieving phrasal density typical of expert academic discourse. Joint analyses reveal that lexical sophistication co-develops with phrasal complexity, reflecting integrated linguistic growth supporting advanced argumentative competence. Qualitative data illustrate how learners’ metalinguistic awareness and strategic writing behaviors underpin these gains. The study contributes valuable theoretical and pedagogical insights for EAP curriculum design targeting rapid, proficiency-sensitive academic writing development in expanding-circle language contexts.
Bahran et al. (2025) studied this question.