Isotactic polypropylene (iPP) was mechanically degraded in the absence and presence of a phenolic antioxidant at liquid nitrogen temperature in the presence of oxygen. The interaction of the primarily formed iPP macroradicals with oxygen and phenol was studied by EPR spectroscopy in the temperature region between 77 and 303 K at antioxidant concentrations of 0.1, 0.5, and 3.0 wt%. It was found that iPP macroradicals react at low temperature with oxygen, yielding peroxy radicals, and with antioxidant, with a simultaneous formation of phenoxy radicals. The development and decay of all kinds of radicals was studied. The rate constants for peroxy radical decay were determined and the apparent activation energies for the regions of slow and fast decay were calculated. © 1993 John Wiley & Sons, Inc.