A contributory factor to the failure of correlation between outdoor and indoor photodecomposition tests is the difference in the ultraviolet spectral energy distributions of natural sunlight and the various indoor test sources which do not duplicate sunlight and differ widely among themselves. The ultraviolet spectral energy distributions of natural sunlight and of a variety of indoor exposure test sources have been measured. By use of a ferrioxalate actinometer, the measurements have been put on an absolute basis permitting the intercomposition of the sources. The xenon arc was found to be the best approximation of sunlight in the ultraviolet. For shorter-wavelength ultraviolet (below 3500 A), the combination of fluorescent Sunlamp and Blacklight lamps also approximated sunlight.