We investigated the collaborative writing actions carried out by 60 Open University of Israel graduate students as they built a wiki glossary of key course concepts. These actions were analysed using a taxonomy of collaborative writing actions (i.e. adding, editing and deleting information) in order to find out what students do and what they do not do when writing collaboratively. Two main findings were reported: in accord with previous research, students most frequently add content to a wiki rather than delete existing text; and contrary to previous research, students modify existing texts to a greater extent than previously reported. These findings may help teachers design collaborative learning activities. Teachers should be aware of the difficulties faced by students when writing collaboratively and should design collaborative learning activities in ways that overcome or circumvent these difficulties. (Contains 3 tables and 3 figures.)