A model for photodegradation of transparent : thermoplastics has been developed and tested using data from polystyrene samples. The proportionality between incident energy and amount of degradation is calculated at each wavelength to yield the "wavelength sensitivity spectrum" for degradation of the polymer. The formal derivation of the wavelength Sensitivity spectrum is described. The model was tested by exposing polystyrene samples containing several different stabilizers at various loadings to two different sources of xenon are ultraviolet radiation. The degradation rates predicted by the model correlated well with the experimentally observed rates over all the different conditions. The model systematically underestimates the absolute degradation. Measurement of stabilizer concentration as a function of depth into the molded test specimens showed that the stabilizer was distributed uniformly before exposure to the light source. After exposure, the stabilizer was largely depleted from the surface layer, but the concentration was unchanged in the bulk of the specimens.