The photo-stabilizing effect of 2，2，6，6-tetramethyl-4-hydroxy-piperidine (TMP) and 2，2，6，6-tetramethyl．4．hydroxy-piperidine-N-oxyl (TMPO) alone and with 2-hydro-xy-4-oetyloxy．benzophenone(uv-531) on polypropylene (PP) films has been investi-gated．Expermental results show that both TMP and TMPO．synergise with uv-531．It is shown also that the decrease in concentration of uv-531 obeys first-order kinetics in films containing uv-531 alone and with TMP and TMPO and the rate constants in cases of films containing uv-531 with TMP and TMPO·are less than that of films containing UV-531 alone．Fronl these facts it has been suggested that the reason of synergism of studied stabilizers consists m ainly in that N-oxyls can scavenge active radicals pro．duced in photooxidation of PP films and inhibite effectively the formation of carbonyl compounds and hydroperoxides and thus protect uv-531 from．photo-des-truction during the photo．oxidation of PP．