The photochemical transformations of the substituted allyl radicals =Si(CH2)(m)CH=CH=CH2 (m = 0, 1, or 3) and =SiO(CH2)(m)CH=CH=CH2 (m = 0-3) grafted onto a silica surface were studied. The cyclization of the allyl-type radicals resulting in the formation of the corresponding P-substituted cyclopropyl radicals =Si(CH2)(m)-c-Pr-. or =SiO(CH2)(m)-C-Pr-. (c-Pr = cyclopropyl) was found to be the primary photochemical process occurring upon photolysis at lambda greater than or equal to 370 nm. Further transformations of the resulting cyclopropyl radicals depend crucially on the m value. In the case of m = 0 or 1, the thermal isomerization of the cyclopropyl-type radicals yielding the parent allyl-type radicals is the main reaction pathway at room temperature. For m = 2 or 3 the reaction of intramolecular hydrogen atom transfer becomes the predominating process. This reaction results in the corresponding alkyl-type radicals =SiCH(CH2)(m-1)-c-Pr-. or =SiOCH(CH2)(m-1)-c-Pr formation. It was shown that the presence of electron-withdrawing substituents in beta-position resulted in the decrease in the thermal stability of the cyclopropyl-type radicals, The experimental data obtained were compared with the results of quantum-chemical calculations.