A static, volumetric method has been used to determine the adsorption equilibrium of CH4 and CO2 and their binary mixtures on 13X molecular sieves at various temperatures between 273 and 343 K. Pressures for the pure component data extend up to 10 bar, while all binary data were obtained at 4 bar and 6 bar. The measured pure isotherms were regressed using different isotherm equations and the regressed parameters were applied to different thermodynamic models such as ideal adsorbed solution theory (LAST), vacancy solution theory (VST) and real adsorbed solution theory (RAST). CH4/CO2 system that deviate from ideality is not well represented by IAST and VST, whereas RAST which include activity coefficient in the adsorbed phase as characterizing parameter for non-ideality, show a much better representation of the binary equilibria. Experimental selectivities CO2/CH4 range from 26 to 2 at different pressure and temperatures. It is concluded that zeolite 13X suitable for natural gas industrial separation with y(CO2) <0.2 at 303 K and 4 bar. For landfill gas upgrading, where y(CO2) <0.45, zeolite 13X can be successfully applied at 303 K, 4 bar and 303 K, 6 bar and 323 K, 4 bar. (C) 2016 Elsevier B.V. All rights reserved.