Selective hydrogen-atom-abstraction reaction by an H or D atom has been studied in a neo-C5H12–C2H6 (less than 1 mol%) mixture at 77 K by ESR spectroscopy. The H (or D) atom produced by the photolysis of HI (or DI) reacts with neo-C5H12 and C2H6 to form neo-C5H11 and C2H5 radicals. In order to obtain H atoms with different kinetic energies, the photolysis was performed with different lights of 313, 254, and 229 nm. The selective formation of the C2H5 radical by the reaction of the H (or D) atom with C2H6 becomes more effective with the decrease in the energy of the H (or D) atom. The formation of the neo-C5H11 radical by the reaction of the H (or D) atom with neo-C5H12 becomes more effective with the increase in the energy of the H (or D) atom.