CSCW systems introduce new requirements for access control, which cannot be met by using existing models. In this paper, a new role-based access control model, RBCSAC (role-based collaborative systems access control), is introduced to meet these requirements. This model formally describes the relationship between the key elements of access control such as data, operation, privilege, role and user. It provides the method for recording access control information. The model grants and revokes access privileges of cooperative users by assigning them some roles and canceling their roles. Two role-assignment rules are also provided and two operation legality checking rules. RBCSAC model is brought forward aiming at accommodating with the characteristics of collaborative systems such as multi-user, interaction, collaboration, real-time, dynamic. This model can meet the requirements for access control in CSCW systems adequately.