All-day experimental tests were performed to determine the thermal performance of two commercial integral storage collectors for solar domestic hot water systems. These tests were performed under a variety of ambient conditions and irradiance levels, both with and without forced circulation and noontime hot water draws. An analytical model was developed to predict the thermal performance of one of the two systems tested and predicted performance was compared with experimental results. Experimental and analytical results indicate that thermal stratification has a mininal effect on the daily collection efficiency of integral storage collector, so that a standard test similar to ASHRAE Standard 93-77 might reasonably be used to obtain the performance characteristics of the collection element of these systems. The results of an ASHRAE 93-77 type test might then be used to obtain performance ratings under ASHRAE Standard 95 procedures using an in-line heat source. The results of the present investigation may be used to validate such an ASHRAE 95 test method.