The effect of nitroxide degradation on polymerization rate and molar mass development in the nitroxide-mediated radical polymerization (NMP) of styrene is studied using electron spin resonance (ESR) spectroscopy and a mathematical model based on the method of moments. The stability of two nitroxides, 3,3-diphenyl-1,1-dimethyl azabutane N-oxide (azabutane) and 2,2,5-trimethyl-4phenyl-azahexane N-oxide (TIPNO), is followed by ESR spectroscopy, and the products present in the final mixture are separated and analyzed. Significant nitroxide degradation is observed in both cases. The rate of azabutane degradation is about 100 times faster than that of TIPNO. Although a degradation mechanism is proposed, it is assumed in the model, for simplicity, that a new type of radical and a nitroso compound are produced. This new radical can propagate and terminate. Its participation is properly addressed in the polymerization scheme and its corresponding model. The model is validated using experimental data of styrene polymerization mediated by alkoxyamines derived from azabutane and TIPNO at 120 degrees C. Azabutane degradation significantly affects polymerization rate, initial molar mass, and end-group functionality of the polymer. The model provides a good representation of the effect of nitroxide degradation on NMP of styrene.