Chemical changes occuring in thermoset polyesters during exposure to solar and carbon-arc radiant energy were determined quantitatively by a color reaction with a diamine, N,N-dimethyl-p-phenylenediamine, in a methanol-benzene solution. The amount of diamine reacting with a specimen and the color change of the treated specimen, as measured by differential reflectance, varied directly with exposure time. Three commercial polyesters (glass fiber-reinforced) exposed under identical conditions showed widely different degradation rates as measured by the amount of N, N-dimethyl-p-phenylenedimine used per specimen. The relative stability of the three materials was independent of irradiation source. The color of the treated specimen was dependent on exposure time.