Adsorption of carbon dioxide(CO2) and methane(CH4) in faujasite (FAU)-type zeolites NaX and NaY was studied by performing the grand-canonical Monte Carlo(GCMC) simulations at 288,298 and 308 K and a pressure range upto 10 MPa. Simulation results have been analyzed using Langmuir and Toth model. The latter provides a better description of CH4 and CO2 adsorption with respect to the former suggesting that NaX and NaY present a heterogeneous surface in the adsorption especially for CO2. Thermodynamic parameters of Gibb's free energy change, enthalpy change, and entropy change were calculated using adsorption equilibrium constant obtained from the GCMC simulations. The results suggested that NaX has higher affinity for both CH4 and CO2 than NaY and it is more favorable for CO2 than CH4 to adsorb in NaX and NaY. Although NaX has higher affinity than NaY, NaY exhibits higher maximum adsorption capacity due to the volume effect of sodium cations at high pressure. Adsorbed molecules are in ordered arrangement in both NaX and NaY and they have more orderly arrangement in NaX than in NaY. This study provides a quantitative evaluation of CO2 and CH4 adsorption in FAU-zeolites. Crown Copyright (C) 2014 Published by Elsevier Ltd. Allrights reserved.