The intensive photostabilizing activity of hindered amine light stabilizers (HALS) is connected with the presence of N-oxyl radicals formed from them during photooxidation. Their ESR signal in polymers, however, is a superposition of the symmetrical triplet of the mobile N-oxyl radical and an asymmetrical signal of this radical being in a restrictive environment. In oxygen-poor atmosphere, the ESR signal of a tertiary hindered amine is somewhat different from that of a secondary one. Concentration changes of N-oxyl radicals and interaction between a HALS compound and a hindered phenol were investigated. It has been stated that the HALS exert an antioxidant effect, but the phenolic antioxidant hinders the photostabilizing effect of the HALS compound. This is presumably due to a direct reaction between the N-oxyl radical and the hindered phenol.