The inhibitory processes operating when a 2(-2′-hydroxyphenyl) benzotriazole-based ultraviolet absorber (UVA) and a hindered nitroxide free radical are applied to high-yield pulps were studied using milled wood lignin and filter paper as a model. Using quantitative ³¹P NMR it was determined that the UVA is almost completely photostable during irradiation, suggesting that its protective mechanism is based primarily on ultraviolet absorption. Furthermore, the nitroxide was found to protect the UVA from photodegradation. Analysis of irradiated lignin samples involved derivatization followed by reductive cleavage of β-arylether groups. The phenolic-OH groups thus released were quantified using ³¹P NMR. The benzotriazole/nitroxide stabilizing system was found to cooperatively inhibit the cleavage of β-arylether groups. Furthermore, this system was found to have a synergistic inhibitory effect on the formation of catechol structures. These results suggest that the synergism observed between benzotriazole-based UV screens and nitroxyl radicals on the inhibition of yellowing could be a result of their ability to slow the formation of catechol structures and β-O-4 cleavage in addition to the possibility of a UVA-regeneration mechanism.