The National Bureau of Standards conducted outdoor durability tests on eight different types of commercially available flat plate solar collectors. The test results for five types of collectors are analyzed. The purpose of the test program was to investigate the feasibility of determining the durability of materials by measuring collector thermal efficiency at specified intervals. The important material properties include the solar absorptance and long wave emittance of the absorber surface, solar and long wave transmittance of the cover material and the thermal conductivity of the insulation. Tests were conducted by three independent testing laboratories located in Phoenix, Arizona, Cape Canaveral, Florida, and Palo Alto, California. The test sites were chosen to investigate the effects of the various environmental conditions found in the United States on collector degradation. Three test series were considered to study the effect of various operating conditions on collector degradation. The collectors were exposed to the environment for up to 240 days. The thermal performance test results did not reveal significant degradation in the performance parameters, F_r(T_α) and F_rU_L, of the collectors considered. Any degradation that may have occurred was overshadowed by experimental uncertainty. The test results did not depend on test series, geographic location or season of the year. Referring the test results to a common set of weather conditions did not reduce the scatter in the results. / Master of Science