Based on a modified form of the Gibbs equation, two conditions of thermodynamical correctness have been defined. According to these conditions the change in the free energy of the surface-calculable from all isotherm equations-must have finite limiting values at THETA --> 0 and THETA - 1. It has been proved that most of the known and widely used isotherm equations are, from a thermodynamic standpoint, incorrect. These incorrect relationships can be transformed into correct ones with the help of a differential equation including the change in the relative free energy of the surface. The transformation mentioned above modifies the original physical models of physical adsorption. (C) 1994 Academic Press, Inc.