Adsorption isotherms for the pure gases methane and carbon dioxide and their binary mixtures at constant gas composition have been determined in a high pressure microbalance at 20, 40 and 60 degrees C and equilibrium pressures up to 1.7 MPa on activated carbon. The gravimetric method proposed by Van Ness has been used to obtain the composition of the adsorbed phase for the binary mixtures at 100 and 530 kPa. The ideal adsorbed solution (IAS) theory has been applied to predict the binary adsorption equilibria from the single-component isotherms. The predictions of the IAS theory are only in moderate agreement with experimental data. The activity coefficients calculated for both components in the adsorbed phase are analogous to the respective vapour-liquid equilibria positive deviations from Raoult＇s law. Finally, the real adsorbed solution (RAS) theory has been used to fit the experimental data.