The photochemical degradation rates of two solid-state polymer systems, a cross-linked poly(vinyl chloride) and a segmented polyurethane, that contain metal-metal bonds in the backbone were found to exhibit biphasic behavior. The polymers initially degraded quickly but then degraded slower at longer irradiation times. To investigate the origin of this biphasic rate phenomenon, the data were fit to four kinetic models: first- and second-order rate equations, a simple-diffusion model, and a diffusion-controlled model. None of the models adequately described the data. A new kinetic model was developed to describe a biphasic photodegradation rate that was inspired by Perrin's model of fluorescence decay rates. Using this new Perrin-like model, the derived rate equation fit the data very accurately. The resulting fitting parameters obtained as a function of reaction temperature were also consistent with the model. To investigate the versatility of this Perrin-like model, literature data for the irradiation of polyoxymethylene were also, evaluated. It was found that this system also exhibited Perrin-like kinetics.