A plot of the excess surface work that is the product of the adsorbed amount and the change in chemical potential versus the adsorbed amount yields a minimum at the monolayer capacity which is attributed to the first adsorbed molecular layer. The second parameter obtained is the change in chemical potential where adsorption sets in, An integral evaluation gives a linearized form from which both parameters can be found from slope and intercept. The model can be used for adsorption from gas and liquid phases. Regarding the precision and range of validity, the model is an improvement on other methods, especially the BET method.