The advent of technology has facilitated the study of language development and writing development in the form of learner corpora. While learner corpus studies have flourished in recent years, few consider evaluative language development. This paper reports on a study which examines the use of evaluative that-clauses, a linguistic structure that is regularly used to express evaluation in academic writing, in a longitudinal corpus of 304 argumentative essays written by a group of undergraduate students at a university in Brunei. Results suggest students' dynamic use of language resources over time, and support the findings of previous research on the use of evaluative that-clauses by undergraduate students in other contexts of learning. This study, based on an approach to treating learner language in its own right, contributes to the understanding of the nature of language development. Implications for language teaching, including a revised role for teacher feedback and the use of longitudinal learner corpora for students' learning, are considered.