The steady-state and time-resolved CIDNP and flash photolysis methods were used in a detailed study of the photo-Fries rearrangement of 1-naphthyl acetate (I) in acetonitrile and methanol. The main reaction channel is the decay of I through the excited singlet state with the quantum yields 0.17 ± 0.02 in acetonitrile and 0.42 ± 0.04 in methanol at room temperature. The absorption spectra of the naphthoxyl radical and triplet state of 1-naphthyl acetate were detected. The quantum yield of triplet was estimated as 0.4 ± 0.2 and 0.35 ± 0.17 in acetonitrile and methanol, respectively. It has been established that the triplet-born radical pairs make a main contribution to the CIDNP of the photo-Fries rearrangement products. The involvement in the process of two different triplet states of I was supposed. The main decay channel of the lowest triplet state is the triplet−triplet annihilation, while the CIDNP of photo-Fries rearrangement products results from the decay of the upper triplet state of I with a lifetime of a few nanoseconds. The kinetics of CIDNP formation in reaction products has been analyzed, and the rate constants of the rearrangement of the preceding intermediates at room temperature have been estimated.