Most of the classic gas/solid isotherm equations are inconsistent with the general rules of surface thermodynamics. The mean cause of this inconsistency is the fact that, according to the exact thermodynamic relationships, if any calculations or considerations are made, the excess amount adsorbed on the surface should be taken into account. On the contrary, the classic isotherm relationships are calculated with the absolute amount adsorbed, that is, they neglect the differences between the densities of the gas-phase and the adsorbed-phase. This neglection leads to contradictions, especially at high equilibrium pressures when the classic equations calculate the total adsorption capacity by extrapolation of the equilibrium pressure to infinity. However, under the critical temperature (T-c) this extrapolation is a physical nonsense and there are some conditions at which this extrapolation above T-c is also irreal. All these problems can be solved if the classic isotherm equations are transformed to modified forms including a new parameter chi, which assures in every respect the thermodynamic consistency (correctness).