Research has demonstrated that collaborative testing, working on tests in groups, leads to improved test scores but the mechanism by which this occurs has not been specified. Three factors were proposed as mediators: cognitive processes, interpersonal interactions and reduced test-anxiety. Thirty-three students completed a multiple-choice exam individually and in two-person collaborative groups. Test anxiety was reduced but was not directly related to increased exam scores. Improved exam scores were directly related to three cognitive processes: remembering information better, an increased ability to think about the information being tested and having good discussions. Both direct and indirect paths between cognitive variables and enhanced test outcomes were found. Good interpersonal interactions were independently related to satisfaction with the collaborative process. (Contains 3 tables.)