Pure component adsorption equilibrium of CH 4 and CO 2 on activated carbon have been studied at three different temperatures, 298, 323, and 348 K within a pressure range of 10-2000 kPa. Binary adsorption equilibrium isotherm was described using extended Sips equation and ideal adsorbed solution theory (IAST) model. Experimental breakthrough curves of CO 2 /CH 4 (40:60 in a molar basis) were performed at four different pressures (300, 600, 1200, and 1800 kPa). The experimental results of binary isotherms and breakthrough curves have been compared to the predicted simulation data in order to evaluate the best isotherm model for this scenario. The IAST and Sips models described significantly different results for each adsorbed component when higher pressures are set. These different results cause a significant discrepancy in the estimation of the equilibrium selectivity. Simulated and experimental equilibrium selectivity data provided by IAST presented values of around 4, for CO 2 /CH 4 , and extended Sips presented values of around 2. Also, simulated breakthrough curves showed that IAST fits better to the experimental data at higher pressures. According to the simulations, in a binary mixture at total pressure over 800 kPa, extended Sips model underestimated significantly the CO 2 adsorbed amount and overestimated the CH 4 adsorbed amount.