The aim of this work is to predict the adsorption of pure-component and binary mixtures of methane and carbon dioxide in a specific activated carbon, A35/4, using grand canonical Monte Carlo (GCMC) simulation. Methane is modeled as a one-center Lennard-Jones (LJ) fluid and carbon dioxide as a two-center LJ plus point quadrupole fluid. Experimental adsorption data for the system have been obtained with a new flow desorption apparatus. The pore size distribution (PSD) for the carbon was determined from both of the experimental CH4 and CO2 isotherms at 293 K. To extract numerically the PSD. GCMC-simulated isotherms for both pure components in slit-shaped pores ranging from 5.7 to 72.2 Angstrom were used. Using only pure experimental CO2 isotherm data, it was not possible to determine a PSD that allowed a reasonable prediction of the pure methane adsorption. However, with both experimental data sets for the pure components, it was possible to derive a PSD that allowed both experimental pure-component isotherms to be fitted. With this PSD and the simulated adsorption densities in single pores, it was possible to predict in good agreement with experiment (i) the adsorption of binary mixtures of CO2 and CH4 and (ii) the adsorption of both pure components at higher temperatures. However, the model was unable to reproduce precisely the experimental pressure dependence of the CO2 selectivity.