In recent years, several electron spin resonance (ESR) studies of mechanical degradation of crystalline polymer solids initiated by grinding [1,2], fracture, and mechanical stresses [3-7] have appeared. Highly oriented fibers or films were used in the mechanical loading experiments. The general consensus is that under high stress the most strained chemical bonds in polymer chains are cleaved and macroradicals are formed. Each chain rupture produces two radicals which under favorable conditions may be observed by ESR. The presence of air (oxygen) or high chain mobility enhanced by high temperature and swelling agents can so efficiently destroy the radicals that the radical population never reaches the sensitivity limit of the ESR spectrometer. In the absence of such scaveging processes the primary radicals produced by chain reupture may still be rather unstable and tend to transfer their energy to other close-by groups. By such transfer, eventually very stable secondary radicals are formed which are easy to observe. Unidentified impurities (most probably antioxidant) in nylon 66 fibers, investigated by Campbell and Peterlin [5], seemed to serve as such secondary radical centers.