A rigorous theoretical development of the Potential Theory Approach for predicting mixed-gas adsorption on solids from single-gas adsorption isotherms is given. That approach requires very simple computer calculations making it very attractive for process design and control in industrial applications of mixed-gas adsorption on solids. It is shown that the "coalescence" operation can be treated as a purely numerical operation not involving the knowledge or assumptions about some physical quantities the meaning of which is not clear. However, it is also shown that the traditional coalescence operation is not the best way to elucidate the values of the parameters which are necessary to predict mixed-gas adsorption equilibria. A more effective way to determine the values of these parameters better, based on a theoretical analysis of single-gas adsorption isotherms, is proposed.