The present study investigates how the linguistic and cognitive characteristics of second language learners' writing change over the course of a one-month intensive English for Academic Purposes (EAP) summer programme at a British university. A mixed methods approach was used in the study in order to obtain a more nuanced picture of the students' writing skills development over time. Data for this study was collected using the relatively new research tool, keystroke logging. The participants (25 postgraduate and 14 undergraduate students) were asked to produce two argumentative essays, at the beginning and at the end of the EAP course. The essays were analysed using measures theoretically motivated by previous research in corpus linguistics, systemic functional linguistics and developmental child language acquisition. All students participated in pre-course and post-course interviews and completed three learning journal entries during the course. MANOVAs, ANOVAs and non-parametric Wilcoxon signed-ranks tests were used to analyse the quantitative data, while the interviews and learning journals were transcribed and coded manually. The quantitative results suggest that despite no explicit focus on lexis and syntax in the EAP programme, by the end of the course the undergraduate and postgraduate students' writing exhibited a developmentally more advanced repertoire of lexical and syntactic choices that are characteristic of expository texts in academic contexts. With regard to writing fluency, controversial results were obtained, the undergraduate students having advanced in terms of their writing speed, whereas the postgraduate students, whose writing speed has decreased, made significantly more content-oriented revisions at the end of their study on the EAP programme. The qualitative results revealed a number of interesting findings. Specifically, the goals that the students set became more focused on academic writing rather than on language improvement. The results of the analyses have also shown that the majority of undergraduate students gained in terms of confidence in their writing skills and their self-efficacy beliefs increased substantially during four weeks. Finally, regarding the writers' difficulties, in contrast with the beginning of the EAP programme, when most students believed that vocabulary constituted the biggest challenge for them, at the end of the course, vocabulary was scarcely mentioned. These findings coupled with the quantitative results indicate that the students showed notable improvement on the EAP programme.