Effective writing requires important features and one of them is cohesion which is necessary to bind a text. Halliday and Hasan (1976) state that grammatical and lexical devices which are called cohesive devices (CDs) create texture. To make a text coherent and understandable, CDs should be used frequently and skilfully. The present study aims to investigate the students' employment of CDs in a specific genre. The participants were 60 EFL students enrolled in the beginner and preintermediate preparatory classes at a state university in Turkey. The student's opinion essays were examined via the modified version of Halliday and Hasan's (2001) taxonomy of CDs developed by Yang and Sun (2012). The collected data were analysed by calculating the frequency of CDs and comparing the means of CDs used correctly or incorrectly by the participants. The outcomes yielded that the pre-intermediate level students used cohesive devices more frequently and accurately in their opinion essays compared to the beginner-level students. In addition, it was diagnosed that the most problematic area for both levels was the use of conjunctions. Hence, as a pedagogical implication, it is recommended that cohesive devices be taught at the core of writing instruction.