Abstract The properties of potassium-carbon intercalation compounds (KC x ) are affected by the nature of the carbon used as a starting material, as well as by the chemical composition of KC x . We have studied isotopic hydrogen gas sorption in KC x which were prepared from different heat treatment temperature (HTT) carbon materials. The partition coefficients between the absorbed phase and the gas phase, which were measured for H 2/D 2 gas mixtures, agreed well with the predicted values from the isotherms of the pure gases. In the case of low HTT carbon compounds, the existence of an intermediate phase between KC 8 and KC 24 was confirmed by considering the isotope effects and the amounts of hydrogen gas absorbed by KC x of various compositions. Furthermore, the estimated standard enthalpy and entropy change of absorption for H 2 or D 2 enabled us to propose an absorption site structure model for the KC x compounds.