This corpus-based study compares L1-English and L1-Chinese undergraduate students' use of lexical bundles in English argumentative essays, and identifies the most common bundle misuses in L2 student writing. Data consist of two corpora of student-produced argumentative essays: 101 high-rated essays written by L1-English students and 105 high-rated essays written by L1-Chinese students. Using Biber's (Biber et al., 1999; Biber et al., 2004) structural and functional taxonomy, we compared the forms and functions of four-word bundles used by L1-English and L1-Chinese university students. Findings indicate that L2 students not only use substantially more bundle types and tokens than L1 writers, but the structural and functional patterns of bundles also differ. While L1 writers' bundles consist of mostly noun and preposition phrases, L2 students use significantly more verb phrase (clausal) bundles. Results also show that L2 student writers use significantly more stance bundles than L1 writers. In addition, most of the misused bundles in the L2 writers' essays pertain to grammatical mistakes, particularly with articles and prepositions. We conclude with some pedagogical implications for ESL composition.