A new group-contribution model is developed to predict adsorption equilibrium of non-reactive gases in porous adsorbent media. In this model, we convert the adsorption equilibrium problem into a traditional liquid-vapor equilibrium problem by assuming that the adsorbent can be described by a virtual chemical (a virtual group), and thus the adsorbate-adsorbent system (an adsorbate-adsorbent solution) can be treated like a liquid mixture. We solve the resulting VLE problem with the popular UNIFAC equation, taking advantage of the readily available UNIFAC database for common chemical groups. The UNIFAC parameters for a virtual group are obtained from experimental adsorption equilibrium data on simple molecules. These parameters are then used to predict the adsorption equilibrium of more complex molecules containing the same functional groups. The prediction results for a wide range of pure gases and gas mixtures are satisfactory. The model is particularly useful in predicting the adsorption equilibrium of complex chemicals, on which no data are available, from their molecular structures.