Accurate description of gas adsorption on solids is gaining in importance as the number of engineering applications expands. In the present work, a general two-dimensional equation of state (2-D EOS) is presented and the fugacity equations are derived to describe the adsorbed phase for adsorption from gas mixtures. Model constants in the 2-D EOS are determined from pure adsorption equilibria data and then used to predict adsorption of mixtures on both homogeneous and heterogeneous surfaces. The 2-D van der Waals, Soave-Redlich-Kwong, Peng-Robinson, Eyring as well as a new 2-D EOS are evaluated for systems which include both ideal and nonideal adsorption behaviors. The predictions are compared with those of other methods, including the Langmuir model and ideal adsorbed solution theory. Various forms of the 2-D EOS are found to be capable of representing azeotropic behavior, and they provide overall the best predictive results (when based on information from pure-component isotherms).