This article reports on a study that comparatively investigated the differences and similarities in the (incorrect) use of cohesive devices by second-year and fourth-year undergraduate Chinese EFL learners in their argumentative writings. Via detailed analysis of the quantitative and qualitative data, this study seeks to reveal if the patterns of variation in participants' (incorrect) employment of cohesive items may help portray whether and how EFL learners of higher proficiency develop their written discourse competence as compared with the lower proficiency counterparts, and to what degree the (correct) use of cohesive devices indicates their writing quality. Results show that EFL learners at different proficiency levels significantly diverged from each other in their (incorrect) adoption of some cohesive items. In the main, higher proficiency EFL learners' overall ability to apprehend and manipulate cohesive devices has indeed strengthened, which to some extent reflects the gradual maturation of their written discourse competence. Meanwhile, it merits heed that the upward trend may not be in an absolutely all-round fashion. Results also demonstrate that the (correct) use of cohesive devices correlated significantly positively with the writing quality, irrespective of the EFL proficiency levels. These findings hold some implications for ESL/EFL writing theory and pedagogy. (Contains 14 tables.)