The article presents a lexical study that investigates the lexical demands of academic written texts at different levels of writing. By employing the British National Corpus/ Corpus of Contemporary American English (BNC/COCA) word list and the Academic Word List (AWL), the present study analyzed data from the British Academic Written English (BAWE) corpus, which contained 2,761 student assignments, and the Public Library of Science One (PLOS ONE) corpus which included 4,000 scholarly articles. Results from the analyses demonstrated significant differences in lexical difficulty between students' assignments and scholarly publications. The proportion of academic vocabulary in written texts was also found to increase as the writers' levels went up. Cross-discipline comparisons highlighted the difference in lexical difficulties between scientific disciplines.