Abstract Studies of the formation and properties of monolayers of di-oleoyl lecithin (di-O-PC) at the mercury/water interface are reported. Experiments have been performed to investigate the effects of temperature, pH, and ionic strength on the differential capacitance-potential curves of the lipid monolayer. The effect of incorporation of cholesterol into the monolayer and the interaction of the di-O-PC monolayer with proteins and different electrolytes has been studied. It has been found that the measured capacitances are characteristic and reproducible. There is a low capacitance region with a minimum value of 1.85 μF cm −2 between −0.15 and −0.85 V vs. Ag/AgCl/3.5 mol dm −3 KCl. The value of the differential capacitance increases at positive and negative potentials and two reversible capacitance peaks are observed in the negative potential region. There is a peak at −1.25 V which appears to represent breakdown of the adsorbed layer. It has been shown that cholesterol incorporation into the layer causes a decrease in the capacitance minimum of the layer up to a mole fraction of 0.6. In the capacitance minimum potential region the monolayer shows no interaction with the water soluble protein, bovine serum albumin (BSA). At potentials negative to the reversible cathodic capacitance peaks, this property is reversed. It is proposed that the capacitance peaks represent a reorganisation of the monolayer at a critical electrical field which can be altered by the binding of cations. The potential of the capacity increase at more positive potentials is sensitive to the anion of the electrolyte in the order in which these anions adsorb onto mercury.