The adsorption of pyrimidine on a mercury electrode from 1 M KF solutions is studied by means of differential capacity, zero charge potential and maximum surface tension measurements. The data are analysed conforming to the Frumkin isotherm and to the Nikitas approach. A value for the size ratio parameter close to one is obtained. The interaction parameters depend on the electrical variable and indicate predominant adsorbate-solvent interactions at the potential of maximum adsorption. The maximum standard Gibbs energy of adsorption is indicative of physical rather than chemical adsorption. The comparison with other diazines, pyrazine and pyridazine, and with pyridine on the basis of the Esin-Markov effect and the electrosorption valency at low coverages allows some conclusions to be drawn about the relative roles played by the electrostatic interactions with the field and the electronic interactions with the metal, and the participation of H-bonds with the solvent.