Abstract The photochemical secondary processes of acetanilide have been studied. The photochemical reaction of p-methylacetanilide at 2537 Å did not produce the p-rearrangement product, but the o-rearrangement one. The quantum yields of the o-rearrangement and of the disappearance of p-methylacetanilide were 0.092 and 0.12 respectively. Comparing the quantum yields of acetanilide with those of p-methylacetanilide, the values of the quantum yields for the predissociation of the N–C bond, the back reaction, and the recombination ratio were estimated to be 0.25, 0.10, and 0.9 respectively. These results indicate that the recombination reaction between the pair radicals formed by the predissociation of the N–C bond occur efficiently in a solvent cage. The rate constants of the back reaction, the o-rearrangement, and the p-rearrangement were proportional to the odd π electron densities of the anilino radical. No intermediate was observed by the flash photolysis. The small dependence of the quantum yields of acetanilide on the temperature supports the predissociation mechanism previously described. On the basis of these studies, the mechanism of the photochemical reaction of acetanilide in the liquid phase has been proposed.