Data taken during the National Bureau of Standards Durability/Reliability (NBS D/R) Program is analyzed in depth in this report. The main thrust of the analysis is to develop methods of relating material property changes to full scale collector efficiency changes after exposure to environmental and operational conditions. To this end, results of material sample and full scale collector tests are discussed separately and then related to each other. Many of the materials tested showed no measurable change in optical properties after 480 days of exposure. Therefore, the results of six representative samples which span the range of responses observed are presented in detail. Next, a mathematical model of collector thermal performance is used to show the theoretical dependence of efficiency on property values. Actual degradation measurements of three typical collectors in the NBS D/R Program are presented. These measurements, based on ASHRAE standard 93-77 tests, show degradation between 0 and 4 percent of the original value of efficiency. Also presented is an analysis of the stagnation temperature variation with exposure time. No conclusive data resulted from the stagnation temperature analysis. Finally, the mathematical model is used with results from the material tests to predict the change in efficiency. The predictions are always within ± 3 percent of the measured 480-day efficiency. Error is estimated to be a result of uncertainty in insulation conductivity values used and other unexplained sources of error. / M.S.