Adsorption of 3-acetylopyridine (3-ACP) at the Hg\solution and air\solution interface was studied by means of differential capacity and surface tension measurements. In the case of the Hg\solution interface, the experimental data were analysed to determine the surface excess and adsorption parameters as a function of the electrode charge density and the bulk concentrations of the solute. The results obtained indicate that 3-ACP molecules, for small concentrations, are adsorbed flat on the mercury surface and for greater concentrations the orientation changes to tilted or even perpendicular. In the latter case however, the adsorbed layer contains mixture of adsorbate and water molecules. Comparing the adsorption parameters for these two interfaces it is evident that the adsorption at the Hg\solution interface is markedly greater than that at the free surface. The role of the pi electrons of the pyridine ring in the adsorption process on mercury was also discussed.