The mechanism responsible for the acid-catalyzed cross-linking of a photoresist based on poly(4-hydroxystyrene) was studied by H-1 NMR spectroscopy. The reaction involving cross-linkers that contain benzylic alcohol groups is proposed to proceed via both a direct C-alkylation as well as an initial O-alkylation followed by a subsequent acid-catalyzed rearrangement to the final C-alkylated product. The study reveals that a thermal cross-linking process is competing to a significant extent with the acid-catalyzed one. Two mechanisms involving different intermediates, quinone methides and carbocationic species, are proposed for these two competitive processes. The mechanistic understanding provided by this study can be used to explain some unusual features of this resist system such as the lack of correlation between the apparent resist sensitivity and the concentration of the cross-linker or the temperature of the reaction.