This study sought to investigate the key determining characteristics in the writing performance of first-year Chulalongkorn University students across language proficiency levels as measured by CU-TEP. The focus was on both syntactic and lexical complexity components. The sample comprised the writings from a corpus of 4,812 first-year students divided into four CEFR levels (C1, B2, B1, and A2), using CU-TEP and corresponding CEFR levels as the strata. The sample size of all four groups was identical comprising 50 students each, totaling 200 students. Multiple computational tools were utilized for data analysis. The findings revealed that the distinctive features typifying the most proficient writers include the production of longer as well as more clausally and phrasally complex sentences. They also demonstrated high lexical richness through the use of wide-ranging vocabulary and rare or sophisticated academic words. These features were also discovered in other less proficient groups but to a lesser extent at decreasing proficiency levels. It was also found that the syntactic complexity measures that better differentiated proficiency levels were: mean length of sentence, mean length of T-unit, and mean length of clause while all three lexical complexity indices were proven to be good predictors of L2 writing quality. [Note: The publication month range (Jan-Jun) shown on the PDF is incorrect. The correct month range of publication is Jul-Dec.]