Photoinduced isomerization of alkyl radicals trapped in gamma-irradiated 77 K solid alkanes has been studied using electron spin resonance method. Alkyl radicals trapped in the crystalline solids of n-alkanes from n-C8H18 to n-C13H28 and in the glassy solids of 2- and 3-methylheptanes have been measured. Interior radicals (-CH2CHCH2-) trapped in the orthorhombic crystals of n-alkanes are converted to primary radicals (CH2CH2-) with irradiation of 254 nm light, whereas the interior alkyl radicals in the triclinic crystals of n-alkanes are converted to penultimate alkyl radicals (CH3CHCH2-) This selectivity of photoconversion is explained by the initial formation of the primary radicals in both the crystals and thermal radical conversion to the penultimate alkyl radicals in the triclinic crystals. Selective photoisomerization to primary radicals (CH2C7H15) has also been observed for penultimate radicals (CH3CHC6H13) trapped in gamma-irradiated 2- and 3-methylheptanes. Excited alkyl radicals trapped in solid alkanes hence initially isomerize to primary radicals regardless of the structure of the solids.