A new simple method has been developed for the estimation of deactivation rate constants in atom transfer radical polymerization using copper catalysts. This method is based on the principle of reverse ATRP and is conducted under an air atmosphere, using molecular oxygen as an oxidant for Cu(I) species. The deactivation rate constants for several different reactive copper complexes were estimated by this procedure, and the results are correlated with those obtained from the analysis of the evolution of molecular weight distribution with conversion and with competitive trapping reactions using nitroxides. The effect of temperature on these rate constants was also studied.