The Freundlich isotherm equation, in the form of theta = KP1/n, was derived by treating the model with the ensemble theory in statistical mechanics as a theoretical tool, the model being set up for the adsorption system with emphasis on two factors, namely the lateral interaction between the adsorbed molecules and the energetic surface heterogeneity. The validity of the derived equation was examined from different aspects. The above statistical mechanical derivation led to the theoretical prediction of the Freundlich isotherm equation being also applicable to the nearly homogeneous surfaces and the expressions of the empirical constants K and n in terms of the microscopic properties of molecules and temperature. With these expressions, the experimentally observed properties of the empirical constants K and n (such as temperature dependence) were interpreted and, in a statistical mechanical sense, the relationship between the macroscopic behavior of the adsorption system and the microscopic properties of molecules, of which the adsorption system is composed, was established. Besides, the expression for the empirical constant n led to the estimation of the relative importance of the above two factors to the adsorption capacity of the adsorption system,