When H atoms are produced by the photolysis of HI or the radiolysis of neo-C5H12 in the neo-C5H12−i-C4H10(2 mol %) mixtures above 40 K, the H atoms react selectively with the solute i-C4H10. The selective reaction is suppressed below 30 K. The reactions have been studied by electron spin resonance and electron spin echo measurements at 4−100 K. The drastic temperature effect on the reaction by H atoms suggests that the H atoms concerned are not hot atoms, but thermal atoms. The thermal H atoms react with the solute alkane by quantum tunneling at low temperature. Temperature effects on the ESR spectra and the spin−lattice relaxation times of the solvent neo-C5H11 radicals and the solute t-C4H9 radicals indicate that local motions of the solute i-C4H10 molecules take place above 40 K, while motions of the solvent neo-C5H12 molecules are severely restricted even at 100 K. It is concluded that the local motions of the solute i-C4H10 assist the tunneling abstraction reaction by H atoms, resulting in the selective reaction with the solute above 40 K.