Monolayers of 1-amino-2-sulfonic-4-hydroxyanthraquinone, 1,2,4-AQASH, have been formed on mercury electrodes by spontaneous adsorption. Films were deposited from solutions containing either quinone or hydroquinone forms where the bulk concentration, C-B, was between 0.1 and 5 muM These films have been used as model systems to probe the effect of intersite separation and redox composition on hydrogen bonding interactions in two-dimensions. Both resonance Raman spectroscopy and surface coverage data indicate that at the open-circuit potential the adsorbates are coplanar with the electrode surface. For surface coverages, Gamma, less than approximately 1.4 x 10(-10) mol cm(-2) (C-B less than or equal to 1.5 muM) monolayers formed from the quinone exhibit close to ideal voltammetric responses with a peak-to-peak splitting of 17 mV being observed. Also, the full widths at half-maximum are 53 and 63 mV for the cathodic and anodic branches, respectively. Capacitance data reveal that the adsorbates interact only very weakly within fully oxidized or reduced films. The free energy of adsorption is larger for the quinone (-35.3 +/- 0.2 kJ mol(-1)) than for the hydroquinone (-33.5 +/-0.3 kJ mol(-1)) monolayer. In contrast to fully oxidized or reduced monolayers, the dependence of the surface coverage as determined from the area under the voltammetric peak, i.e., where the film is of mixed redox composition, on the bulk concentration is best modeled by the Frumkin adsorption isotherm. The interaction parameters,g, are negative for monolayers formed from both quinone (-0.29 +/- 0.05) and hydroquinone (-1.78 +/- 0.22) forms indicating stabilizing lateral interactions. Where the intersite separation is less than approximately 3 Angstrom, i.e., when Gamma exceeds 1.4 x 10(-10) mol cm(-2), the anodic branch of the voltammogram becomes distorted and a current spike is observed. These data are interpreted in terms of lateral hydrogen bonding between the carbonyl and hydroxy moieties when the film is in a quinhydrone form.