Adsorption equilibrium isotherms of CO2, CH4, and mixtures of CO2/CH4 on shale sampled from Nanchuan, southeastern Sichuan Basin, were measured at 278, 298, and 318 K by an accurate gravimetric method. The adsorption equilibrium data of CO2 and CH4 were fitted using both the virial model and the Brunauer-Emmett-Teller (BET) model, and the isotherms of CO2/CH4 mixtures were fitted by an extended BET model. On the basis of adsorption data, the adsorption selectivity factors for CO2 over CH4 (alpha(CO2/CH4)) and thermodynamic parameters were estimated. Nanchuan shale was characterized with a high total organic carbon (TOC), having inorganic minerals and wide pore size distribution ranges. The adsorption heat, negative Gibbs free energy change, and negative surface potential of CO2 are larger than those of CH4, and the entropy loss of CO2 is larger than that of CH4, suggesting that adsorbed CO2 is in a more highly ordered arrangement than CH4 on shale. alpha(CO2/CH4) values at different temperatures are all larger than 2.5.