Many photoinitiators exhibit photobleaching in which the absorbance decreases with illumination time when exposed to light of the proper wavelength. This occurs because the absorption characteristics of the photolysis products are different than the original initiator molecule. Two classes of a-cleavable photoinitiators for which photobleaching is particularly pronounced are aryl phosphine oxides in the 365 nm region of the spectrum, and substituted titanocenes in the 450 nm region. Photobleaching is particularly important for photopolymerization of thick polymer parts and pigmented coatings. In this project, a mathematical model is being developed for the photobleaching of photoinitiators, including multi-wavelength effects. Given the incident light intensity and wavelength as well as the initial photoinitiator concentration, the model predicts the variation of the photoinitiator concentration and light intensity with time and sample depth. The model is based upon the simultaneous solution of the governing differential equations, and the simulation predictions will be compared to experimental results for a number of photobleaching initiators. The importance of diffusional effects and multiple incident wavelengths on the photobleaching rate are being investigated.