Wikis are believed to be a powerful tool assisting the development of constructivist learning environments, as their very nature supports collaboration. However, not much research has been done into the types of collaborative interaction that take place in wikis when used for learning. The main purpose of this study is to explore the factors that affect the ways students collaborate in the wiki environment. To this end, a qualitative exploration of students' perceptions of collaboration in the wiki was carried out among sociology students at university level who used this environment in blended, problem based learning as part of their "English for Specific Purposes" course. The research shows that the wiki promoted many collaborative behaviours among students, such as learning from each other and communicating with the teacher. However, the data indicate that the wiki was less successful in facilitating other types of collaboration, such as communicating with peers and co-constructing products, primarily because the students had the opportunity to collaborate in live meetings and preferred to use "Messenger" and email to communicate. Overall, the results obtained here confirm that the wiki can be used to enhance effective collaboration in a constructivist approach to language learning. (Contains 4 figures.)