Carbon capture from flue gas by adsorption processes requires a suitable isotherm model for use in process simulators. Comparative physical adsorption isotherm models are here tested on an adsorption equilibrium loading data set for Zeolite 13X (Z13X) between 298 and 348 K. Dry flue gas mixture adsorption was found to involve enhanced adsorption of N2 by up to 85% relative to levels of N2 mixture adsorption predicted with pure species parameters. This relative N2 deviation was found strongly dependent upon the amount of adsorbed CO2 and suggested to be caused by optimization of molecular quadrupole interactions in the adsorbate layer. A supplemental isotherm expression dependent upon mixture fitting parameters characterized the phenomenon. Prediction of wet flue gas mixture adsorption on Z13X was tested with different numbers of adsorption sites in the α-cavity and logistic formulations to exclude CO2 and N2 from hydrophilic adsorption sites but without success. Shielding the affinity of Z13X toward coadsorbates using the moisture content in the gas mixture improved regression residuals. This method of sticking parameter adjustment described the influence of adsorbed H2O hydrogen-bonded clusters on CO2 and N2 and may provide a path to humid mixture adsorption prediction through studies of pure H2O in porous materials.