Adsorption equilibria were measured for binary mixtures of carbon dioxide and water vapor on zeolites 5A and 13X using a volumetric apparatus. The experiments were conducted at (0, 25, and 50)degrees C with water loadings of (1.0, 3.4, and 9.4) mol . kg(-1). With an increase in adsorbed water loadings, the loadings of weakly adsorbed CO, decrease appreciably. Pure component data are described well by a multitemperature Toth isotherm. Binary data show a discrepancy with predictions of the ideal adsorbed solution theory (IAST) but are described well by a virial excess mixing coefficient (VEMC) model, which adds corrective terms to the IAST to account for nonidealities. Data measured at lower CO(2) partial pressures and higher water loadings show indications of chemisorption and carbonate formation.