The effects of the u.v. absorbers 2,4,-dihydroxybenzophenone (DHB), Tinuvin 213 and Tinuvin P on the anaerobic photochemistry of poly(4-methoxyacrylophenone) have been investigated, the additives being incorporated into thin films and to solutions of the polymer. All retard the degradation of the polymer, in particular both rates of coloration and random chain scission are decreased. None of the additives acts simply as a u.v. absorber, although all of their long-wave u.v. absorptions overlap the n → Π * absorption of the polymer. Rather, experimental evidence supports the proposition that they are acting as quenchers of the CO triplet. The chain scission data conform to Stern–Volmer quenching, and the coloration data for the solid state reactions is well represented by the Perrin equation. DHB is by far the most effective retarder.