This study investigated the effectiveness and students' perceptions of collaborative writing (CW) in second language (L2). The study involved 38 first year students in two intact classes at a large university in the UAE (United Arab Emirates). One class consisted of 18 students and was considered the experimental group, and the second consisted of 20 students and was considered the control group. In the control group, writing tasks were carried out by students individually; in the experimental group, these tasks were carried out in pairs. The study lasted 16 weeks and involved a pre- and post-test. Writing quality was determined by a holistic rating procedure that included content, organization, grammar, vocabulary, and mechanics. Results of the study showed that CW had an overall significant effect on students' L2 writing; however, this effect varied from one writing skill area to another. Specifically, the effect was significant for content, organization, and vocabulary, but not for grammar or mechanics. In addition, most students in the CW condition found the experience enjoyable and felt that it contributed to their L2 learning. Results of the study are discussed in light of the social constructivist perspective of learning. A number of theoretical and pedagogical implications of the study, and limitations and directions for further research, are presented. (Contains 3 tables.)