The stabilization mechanisms of hindered amine stabilizers (HAS) involve various oxidation products oft;he piperidine structure. Thus, we propose a novel method, based on electron spin resonance (ESR) spectroscopy, to check in situ the consumption of the total piperidyl species (intact HAS and all of its byproducts) in polypropylene (PP) films through photooxidation. First, the concentration of nitroxyl radicals produced upon irradiation in stabilized PP has been measured by direct ESR analysis. Then, the changes of concentration of the overall stabilizing species have been detected by indirect ESR, after conversion of the overall HAS derivatives into nitroxyl-free radicals by exposure of photooxidized PP to peracetic acid vapor at room temperature. Results were compared with those obtained by the conventional Fourier transform infrared method in the particular case of [2,2,6,6-tetramethyl piperidinyl] sebacate, and the reliability of both techniques was discussed. Thus, we assume that indirect ESR experiments consist of a easy, accurate, and very sensitive method to monitor the time evolution of a low-molecular weight HAS-NH concentration in PP upon photooxidation.