Photoinduced isomerization of protiated and deuterated hexyl radicals in 77 K solid n-hexanes was examined. 2- and 3-hexyl-h(13) radicals in n-hexane-h(14) isomerize to 1-hexyl-h(13) radicals with the irradiation of 254 nm light. This isomerization is suppressed by replacing the six hydrogen atoms of the methyl groups with deuterium atoms. The quantum yield for the isomerization of 2- and 3-hexyl-1,1,1,6,6,6-d(6) in n-hexane-1,1,1,6,6,6-d(6) and of 2- and 3-hexyl-d(13) in n-hexane-d(14) is less than 1/16 of that of 2- and 3-hexyl-h(13) radicals in n-hexane-h(13) Photoinduced isomerization of hexyl-h(13) radicals in 77 K n-hexane-d(14) was also examined. 2- and 3-hexyl-h(13) radicals isomerize to 1-hexyl-h(13) radical even in the solid. The isomerization may hence occur through an intramolecular reaction. The large isotope effect may arise in the course of either the isomerization in the excited state or the relaxation process of the excited primary radical.