This report summarizes the Swedish contribution to a round robin test concerning the measurement of thermal efficiency and thermal losses of solar collectors. The test is part of the International Energy Agency (IEA) Solar Collector Testing Programme and concerns the outdoor testing of solar collectors. At the IEA 2nd Expert Meeting at Wiehl/Germany October 21-22, 1976 it was agreed that two collectors would be used in the IEA round robin test programme. The collectors would primarily be used for thermal performance tests outdoors. Countries having a solar simulator would also make indoor/outdoor comparisons on the collectors.