Differential capacity measurements are reported for the adsorption of nicotinamide (NcA) and nipecotamide (NpA) at the mercury electrode in aqueous solutions containing KF as a supporting electrolyte. The experimental data were analysed to obtain the surface excess and adsorption parameters of NcA and NpA as a function of the electrode charge density and the bulk concentration of the adsorbate. The results obtained indicate that NcA molecules are adsorbed flat on the mercury surface. The adsorbed layers for both substances contain mixtures of adsorbate and water molecules. The role of the pi electrons of the pyridine ring in the adsorption process and the charge dependence of the standard Gibbs energy are discussed.