A new theory for the prediction of multicomponent adsorption equilibrium in non-crystalline porous solids is presented: the multispace adsorption model (MSAM). This theory allows for the inherent nonuniformity of the adsorbed phase by describing separately the adsorption in “spaces” close to, and far from, the adsorbent surface. The inputs to the calculation are the pure-species adsorption isotherms and a single parameter characteristic of the adsorbent that can be obtained from very limited mixture data. We evaluate the theory by predicting multicomponent adsorption for a range of carbon adsorbents, making use of our own experimental results and literature data. The MSAM gives quantitative predictions for all the systems studied.