Abstract: Equilibrium adsorption data of methane on Ajax activated carbon were volumetrically measured under a temperature of 268.15~338.15 K and a pressure up to 13.5 MPa. Accessible volumes for methane molecules and critical pore width for methane effective adsorption were introduced; on the basis of these concepts, the mass balance of the adsorbate confined within the adsorption cell was used to determine the relationship among the total amount, excess adsorption amount and absolute adsorption amount. By considering the accessible volume and critical pore width, the excess adsorption amount was obtained by the pore size distribution (PSD) determined by Nonlocal Density Functional Theory (NDFT) calculation. The equilibrium adsorption data were fitted by Toth model through nonlinear regression, which was verified the accuracy in predicting the absolute adsorption amount. The isosteric heat of methane adsorption on activated carbon was over-estimated by using the excess amounts, but underestimated by using the total amounts. The isosteric heat of adsorption should be determined from the adsorption isosteres of absolute adsorption amount when considering the thermal effect in adsorption process.