The equilibrium adsorption amounts of hydrogen isotope gas and the Brunauer-Emmett-Teller (BET) surface area were measured at 77 K with different microporous and mesoporous molecular sieves. The results indicate that good linear relationships are observed between the amount adsorbed and the BET surface area at (0.02, 0.04 0.06, 0.08, 0.10, and 0.12) MPa for microporous or mesoporous adsorbents, which provide proof of the monolayer adsorption mechanism for hydrogen isotopes at supercritical temperatures. Effects of specific surface area and pore size are more important than the surface properties of adsorbents for hydrogen isotope storage with the physical adsorption method. The equilibrium adsorption difference between hydrogen and deuterium is increased with a decrease of pressure. Moreover, the adsorption capacity of hydrogen isotopes on mesoporous molecular sieves is smaller than that on microporous molecular sieves at the same temperature, pressure, and BET surface area.