Photoinduced isomerization of secondary alkyl radicals to primary alkyl radicals in glassy solids was studied by ESR spectroscopy. The efficiencies for 2-, 3- and 4-heptyl radicals in 77 K perdeuterated methylcyclohexane decrease in this order. Molecular orbital calculations indicate that the isomerization occurs through a hydride ion transfer from the methyl groups to the radical site in the lowest Rydberg state of secondary alkyl radicals. The dependence of its efficiency on the position of the radical site can be explained by variation of the negative charge at the methyl groups with the position of the radical site.