Abstract: Adsorption isotherms of H2 and D2 on activated carbon(AC), carbon molecular sieve(601) and carbon nanofibers(CNF) are investigated at the liquid nitrogen temperature. Both the onesite and twosite Langmuir models are used for isotherm calculation. Results indicate that all isotherms of H2 and D2 on AC, 601 and CNF can be expressed well with the twosite Langmuir model. Accordingly, it can be inferred that, while adsorbed on carbonaceous adsorbents at the liquid nitrogen temperature, hydrogen isotope molecules should occupy the multiple types of active sites to form a localized monolayer.