Radical processes are key reactions in the photodegradation of polymers. Electron Spin Resonance (ESR) provides a sensitive tool for measuring these paramagnetic substances. The ultrashort time weathering method developed at Bayer AG measures the concentration of radicals generated in a bulk polymeric sample as a function of irradiation time. In less than three hours, quantification of the number of radicals formed in a polymeric sample when exposed to a specific spectrum of light provides an assessment of the light stability or weatherability of the resin. Correlation studies (predicted ESR to real-time weathering) are underway. Evaluation of the type of radicals and their behavior as a function of temperature provides insight into the degradation mechanism. Addition of special radical-scavengers is not required with this method.