Collaboration can bring multiple benefits to projects. Real collaboration appears, however, complex to implement. So far, there is no scientific agreement on the importance of the formal and relational (behavioural) sides of project collaboration. Through a qualitative study of two collaborating teams in large engineering and construction projects we uncover the roles of these two sides of collaboration and analyse their interplay. We find that relational and formal aspects of collaboration are complementary, yet each has a distinct role. The formal integrative mechanisms provide an arena for relational norms to be established, and for collaborative behaviours to be practiced and implemented. Relational norms transform traditional project management activities into collaborative, integrative ones. We advance theory of collaboration by offering a holistic and at the same time fine-grained view on it, and stress that project teams need to pay equal attention to both formal and relational sides for collaboration to succeed.