Changes of the specific surface energy of gold due to the adsorption of pyridine in potassium perchlorate solution measured by Koesters laser interferometry became nearly independent of time after about 10s. Therefore, the experimental specific surface energies and the related quantities of surface charge and surface excess agreed well with those calculated from chronocoulometry. However, the constant phase element in the double layer impedance indicates further small changes after long times. Since no pH-dependence of the specific surface energy in the presence of pyridine was observed, the pK values of adsorbed and dissolved pyridine must be similar. The effective molar mass of adsorbed pyridine is only M(eff)(py) approximate to 2.5 gmol(-1) due to replacement of adsorbed water by pyridine.