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BP Solar is continually analysing performance data on various PV technologies in a long term test program. Recent data obtained from one of these sites indicate that the seasonal variations observed in amorphous silicon modules and arrays may not be due to thermal annealing and solar spectrum shifts as previously reported in the literature. A Millennia array in Germantown, MD, USA with a high tilt angle exhibits two seasonal variations a year with peaks in the spring and fall coinciding with peaks in the plane of array (POA) irradiance, not with variations in temperature or solar spectrum. Additional data will be presented in support of the hypothesis that the seasonal behavior of some of today's amorphous silicon modules is not due to thermal annealing during high temperature operations.