We report a 2.5-year study of the photolytic degradation of l,3,5-triamino-2,4,6-trinitrobenzene (TATB) with variations in temperature, humidity, and illumination by fluorescent and UV light (254, 312, and 365 nm). The free-radical decomposition product was monitored with electron paramagnetic resonance (EPR). The EPR spectrum of the green powder allowed reliable quantitation with a single peak (FWHM = 29.1 G). The variations in humidity showed little effect in accelerating the degradation of TATB. The only significant temperature effect was noticed at -10 °C, where fewer radicals formed. The radical production rates at -10 °C were some of the highest measured, however, suggesting that the rates under other temperature conditions had slowed, perhaps as a result of extensive conversion of surface molecules to radical species. We show that a substantial amount of radicals can be generated with UV light, and work is ongoing to modify our EPR spectrometer so that TATB can be irradiated in the EPR cavity to measure the initial rates of radical formation.