A likely mechanism of especially large photosensitization for a base-developable poly(phenylhydrosilane) resist (1) in the presence of 3,3＇,4,4＇-tetra(tert-butylperoxycarbonyl)benzophenone (EA1) is discussed in terms of photolysis rates and dissolution rates. The increase in the photolysis rate occurs through photoinduced electron transfers from polysilanes to electron accepters. The fluorescence quenching rate of 1 in the presence of EA1 is almost the same as that in the presence of benzophenone (EA2). However, EA1 does increase both photolysis rates and resist sensitivities, but EA2 does not. The difference between EA1 and EA2 is discussed in terms of back-electron transfer in order to explain the importance of the presence of peroxide moieties. In the case of EA2, photoinduced electron transfers as well as back-electron transfers occur. In case of EA1, the cleavage of peroxide bonds on the benzophenone moiety would effectively inhibit the back-electron transfer to the 1 radical cation. The same phenomena are observed for C-60. EA1 photodecomposes to form compounds bearing carboxylic acids which also facilitate the dissolution of photolysis products of 1 into basic aqueous solutions.