The surface excesses of malonate and succinate are derived from differential capacity, potential of zero charge (pzc) and interfacial tension measurements. When the electrode is very negatively charged, the interphase is described well by a simple electrostatic model assigning different distances of closest approach to the electrode for Na+ (lambda = 1.6 angstrom), succinate (lambda = 1.6 angstrom) and malonate (lambda - 4.4 angstrom). When the electrode is positively or not very negatively charged, malonate and succinate are specifically adsorbed on Hg. In both cases the adsorption is described well by a virial isotherm, having similar adsorption parameters. The adsorption of succinate on Hg when compared with earlier results in the literature on (210) and (311) gold faces follows the sequence Au(311) > Au(210) > Hg. Integral capacities and electrosorption valency are also calculated and analyzed, suggesting that (a) at potentials close to or more negative than the pzc, specifically adsorbed malonate and succinate place their electrical charges in the vicinity of the outer Helmholtz plane and probably do not make direct contact with the electrode, and (b) at potentials more positive than the pzc, succinate progressively replaces water molecules in contact with the electrode and probably lies flat on the metal surface, while malonate retains its carboxylate groups partially oriented towards the solution.