This cross-sectional study compares the written language of Thai EFL students in their first two years of university study. First- and second-year students (N = 170) wrote opinion paragraphs by hand in response to six prompts. Using automated textual analysis tools, clausal (subordination), phrasal (coordinated phrases and complex nominals), and lexical (AWL use and lexical diversity) measures were obtained. Matched-pairs were created by pairing different first- and second-year students from the same faculty of study who responded to the same writing prompt. The results indicated that second-year students produced significantly more complex nominals and AWL words than the first-year students with effect sizes ranging from small to medium. Implications are discussed in terms of pedagogical approaches and assessment in EFL settings, and suggestions for future research are provided.