In this article, we present a new approach to describe adsorption equilibrium of pure gases in a wide range of pressure. This approach is based on a simple statistical mechanics treatment combining the potential theory and lattice fluid models. The obtained equation for the calculation of the excess adsorption can predict the curve progression of isotherms defined by the IUPAC I classification, and for those at super- critical conditions. Notwithstanding that the basic idea of the developed equation is to adapt to the adsorption equilibrium in supercritical conditions at high pressure, the model correlates very well experimental data at low pressure in subcritical conditions. It is applicable to a wide range of pressures and fits satisfactorily the experimental data in a broad range of pressures and temperatures. In particular, the model predicts the maximum of excess adsorption and its minimum. A comparison between this approach and two others is given.