The significant isotope effect on hydrogen elimination reaction from 2,3-dimethylbutane (h-DMB) cations was studied by ESR at 77 K. When a h-DMB-SF6(0.6 mol %) mixture is gamma-irradiated at 70 K, h-DMB(+) ions in addition to h-DMB radicals are produced. When the irradiated mixture is stored at 77 K for 10 min, tetramethylethylene (TME) cations are produced by H-2 elimination from h-DMB(+) ions. The rate constant for the Ha elimination reaction was measured as 1.1 x 10(-3) s(-1) at 77 K. When a 2,3-dimethylbutane-2,3-d(2) (d-DMB)-SF6 (0.6 mol%) mixture is gamma-irradiated at 70 K and then stored at 77 K, TME(+) ions are scarcely produced upon the storage of the irradiated mixture at 77 K even for 5 days. The rate constant for the D-2 elimination reaction from d-DMB(+) ions was measured as 6.5 x 10(-8) s(-1), which is much smaller than the rate constant for the H-2 elimination reaction. The significant isotope effect on the reactions was explained in terms of a model of the tunneling elimination of a hydrogen molecule from a DMB(+) ion.