The sensitivity spectrum of elastomers to sunlight (λ≽300 nm), determined by the variation at 30° C of the initial rate of oxidation or chain scissions as a function of the wavelength of incident light, shows several maxima in the near UV. These maxima are approximately equidistant and the space between them differs only slightly (10 to 11.5 nm) according to the polydienes (polyisoprene, polybutadiene, copolymer butadiene-styrene) in the crude or the vulcanized state, whatever may be the curing system. The identified maxima, existing also under vacuum (10−4 Torr), can be due to the presence of some chromophores in the chains. The latter, in absorbing the light during the primary photophysical process, would initiate the degradation. Among different possibilities, an S0→T1 transition of the isolated double bonds, producing free radicals, seems to be the one likely to explain the results. After the initiation, the photoxidation at 30° C and 5 mW/cm2 proceeds like the thermoxidation near 150° C. For a peroxide vulcanized rubber, the kinetics are similar and the rates of oxidation are somewhat faster under the most harmful sunrays than those observed by Bevilacqua at 120° C. Moreover, the chain scissions are a linear function of the oxygen consumed and their ratio is close to the limiting value of about 6 moles of O2 per broken bond, obtained by this author at 140–150° C.