Photoinduced isomerization of alkyl radicals of n-alkanes trapped in 77 K solids was examined by the ESR method. In crystalline n-alkanes, penultimate radicals (CH3C over dot HCH2-) and interior radicals (-CH2C over dot HCH2-) are converted to primary radicals (C over dot H2CH2-) with irradiation of 254 nm light. This isomerization occurs regardless of the crystal structure. There are isosbestic points in the integrated ESR spectra with different times of the UV irradiation : the ratio of the penultimate to the interior radicals remains constant during the isomerization. This indicates that the interior radicals are directly converted to the primary radicals. The isomerization does not proceed by successive reaction of hydrogen migrations, which mechanism may cause the formation of the penultimate radicals. Photoinduced isomerization from the penultimate radicals to the primary radicals was observed for alkyl radicals in methanol-d(4) glass. The photoinduced isomerization may hence proceed by intramolecular reactions and does not require intermolecular abstraction of a hydrogen atom from an adjacent alkane molecule.