This paper characterizes and compares the degradation observed in thin-film module performance. Three commercially available thin-film modules comprising a-Si:H, a-Si:H/aSiGe:H/a-SiGe:H and CuInSe2 technologies were used in this study. After an initial indoor assessment the modules were deployed outdoors and periodically taken down for indoor assessment. Results obtained indicate that the a-Si modules degraded by the classical Staebler-Wronski effect. The CuInSe2 module, though known to have long-term performance stability, also degraded in this study. The CuInSe2 module showed shunting behaviour before outdoor exposure. This shunting behaviour was enhanced when the module was deployed outdoors under open-circuit conditions. A comparison of the modules' performances outdoors indicates that the low bandgap CuInSe2 material performs best at high air mass values. This paper emphasizes the importance of being able to analyze module degradation. (C) 2002 Published by Elsevier Science Ltd.