Understanding the effect of moisture in an adsorbent that is selective to CO2 over N2 is central to the design and development of adsorption technology for CO2 capture and concentration from power plant flue gas. Molecular simulations of wet flue gas adsorption on Zeolite 13X were here performed in the grand canonical (μVT) ensemble using the Monte Carlo technique in atomistic detail. The generated multicomponent isotherm data spanned the complete gas mixture composition range for adsorbing species CO2, N2 and H2O from 25 °C to 75 °C at 1atm. The adsorption simulations consisted of faujasite zeolite crystal structures with a fixed Si/Al ratio of 1.31, Na+ cation mobility and beta sodalite cage blocking of CO2 and N2 using 4 Å radius virtual blocking spheres. Simulated equilibrium isotherm data demonstrated that the presence of even small amounts of water vapor in the gas mixture has a significant impact on the adsorbate loadings for the remaining gas components in Zeolite 13X. Structural analysis with radial distribution functions revealed a shift in CO2 adsorption away from the framework structure towards α-cavity pore centres and exclusion from sites adjacent to Na+(II) when H2O is present in the gas mixture. A degree of competitive adsorption of CO2 at Na+(III) sites persists at up to 15% relative humidity (RH) at 298 K (0.5 mol% H2O) with significant lateral adorbate-H2O interactions but exclusion beyond that threshold. Lower CO2 loadings were associated with the growth of hydrogen bonded clusters with major changes complete by RH = 20% at 298 K.