The assessment of oral language is now quite commonly done in pairs or groups, and there is a growing body of research which investigates the related issues (e.g. May, 2007). Writing generally tends to be thought of as an individual activity, although a small number of studies have documented the advantages of collaboration in writing in the second language classroom (e.g. DiCamilla & Anton, 1997; Storch, 2005; Swain & Lapkin, 1998). Particularly in university contexts, group or pair assignments are widely used in many disciplines. In addition, collaborative writing could be used in second language classroom assessment contexts as formative assessment. However, research which compares texts produced by learners collaboratively to texts produced individually, and the implications of this for assessment practices, is rare. This study is a first step in the investigation of using collaborative writing in second language contexts and comparing the performance of two groups of second language learners: one group worked individually, and the other group worked in pairs. When writing in pairs, each pair produced a single text. All participants completed one writing task: an argumentative essay. The performances of the individuals (N = 48) and the pairs (N = 48) were compared on detailed discourse analytic measures of fluency, complexity and accuracy. This comparison revealed that collaboration impacted positively on accuracy, but did not affect fluency and complexity. A detailed analysis of the pair transcripts recorded during the writing activity provides insights into the ways in which pairs work together, and the foci of their endeavour. The implications of these findings for in-class assessment of second language writing are discussed. (Contains 8 tables and 2 footnotes.)