An electron paramagnetic resonance (EPR) method for monitoring the rate of atomic bond rupture in rubber subjected to uniaxial tensile strain in an ozone environment is described. The free radicals so formed are found to be quite stable and present in large, easily detectable concentrations. The initial rate of bond rupture was found to be approximately proportional to ozone concentration at fixed strain. Below a prestrain of three percent, no effect of the ozone environment was detected; above this threshold, the rate of rupture increases with strain up to strains of approximately 20 percent. It appears that this EPR technique is very sensitive and can yield information not readily obtainable by more conventional techniques.