The decomposition rate constants k(dec) as well as the activation rate constants k(act) of some alkoxyamines were determined and compared with regard to their structures. The results showed that decomposition depends predominantly on steric factors, while both steric and polar factors affect k(act). Bulky nitroxides are less susceptible to decomposition probably due to the sterically congested transition state in this reaction. Mechanistically, decomposition was demonstrated to occur bimolecularly. (Namely, the unimolecular pathway is insignificant.) It was also found that alkoxyamines have more chances to undergo decomposition at the deactivation stage than at the activation stage.