One important strand of research in collaborative writing has been the learners' attitudes and perceptions toward collaborative writing. This study sets out to compare three pairs of students' perceptions and their actual collaborative writing practices. Multiple sources of data were collected. Students' semi-interviews were audio recorded, transcribed and analyzed for their perceptions. Their collaborative writing tasks are also recorded and analyzed for quantity, type and resolution of language related episodes (LREs). The study further examined the collaborative texts using both quantitative and qualitative measures for students' language improvement. Our finding suggest that most participants expressed positive attitudes towards collaborative writing but only half of them were aware of language improvement. An analysis of pairs' discussion revealed that participants were overly concerned with vocabulary and all LREs were successfully resolved. The quantitative and qualitative analysis of collaborative texts demonstrated that two pairs received considerable language improvements in terms of lexical diversity, syntactic complexity, fluency and text quality. These findings can be used to encourage students to reflect on their own perceptions and practices in collaborative writing tasks.