Different site energy distribution functions based on the condensation approximation method are proposed for the liquid-phase or gas-phase adsorption equilibrium data following the Fritz–Schlüender isotherm. Energy distribution functions for the four limiting cases of the Fritz–Schlüender isotherm are also discussed. The proposed models are successfully applied to the experimental equilibrium data of nitrogen molecules at 77 K on a pitch-based activated carbon (PA) and a pitch-based activated carbon containing boron (PBA). An energy distribution function based on FS isotherm containing five parameters suggest a unimodal distribution of binding sites for carbon PA, the binding site energies being distributed as exponential or unimodal, depending on the pressure, in the case of carbon PBA. The advantages of the proposed models are discussed.