A new algorithm is suggested for the calculation of energies and fractions of adsorption sites of different kinds from adsorption isotherms. The method is recommended for the investigation of crystalline surfaces when the number of distinguishable adsorption sites is not too large (similar or equal to 1-5). The algorithm was applied in the treatment of the adsorption isotherm of nitrogen on the surface of maximally hydroxylated silica at 78 K. The procedure, which allows one to calculate the isosteric heat of adsorption with a correction for effects of multilayer adsorption, is used for calculation of the energies of adsorption.