The photolytic products and a new photoreduction mechanism of benzophenone (BP) as a photoinitiator in the photocrosslinking of polyethylene (PE) and its model compounds (MD) have been studied by means of fluorescence, ESR, C-13 and H-1 NMR spectroscopy. The fluorescence spectra from the PE and MD systems demonstrate that the main photoreduction product of BP (PPB) is benzpinacol formed by the recombination of two diphenylhydroxymethyl (K-.) radical intermediates. The ESR spectrum obtained from the UV irradiation of the MD/BP system gives positive evidence of K-. radicals. Two new PPB products: an isomer of benzpinacol with quinoid structure, 1-phenyl-hydroxymethylene-4-diphenyl-hydroxymethyl-2,5-cyclohexa-diene and three kinds of alpha-alkylbenzhydrols have been detected and identified for the first time by C-13 and 1H NMR spectroscopy from the MD systems. The latter could be formed by the reactions of K-. radicals with alkyl radicals produced by hydrogen abstraction of the excited triplet state (3)(BP)* from polyethylene or its model compounds. These results provide new experimental evidence for elucidating the photoreduction mechanism of BP in the photoinitiated crosslinking of polyethylene.