The electrochemical properties of phospholipid monolayers of dioleoyl phosphatidylcholine (DOPC) spread from the gas/solution interface on mercury surfaces are quantitatively investigated in this paper. These layers display multiple states which interconvert through phase transitions characterized by two sharp capacitive peaks. Potential pulse techniques (chronocoulometry) were used to quantitatively investigate the properties of the DOPC monolayer on a mercury electrode. Charge density and the resulting film pressure due to DOPC spreading at the Hg/solution interface were determined. Results indicate that the lipid layers are displaced from the mercury surface at negative potentials in excess of -1.8 V. The potential of maximum film pressure or stability of the lipid monolayer and the shift in the potential of zero charge due to lipid transfer to the mercury surface were estimated as -0.4 and +0.435 V versus Ag/AgCl (saturated KCl), respectively. The similarity of the DOPC monolayer properties on mercury to the insoluble surfactant monolayer properties on single-crystal gold electrodes is noted. The spread DOPC layer and specifically the first phase transition was further characterized utilizing Tl+ and Cd2+ reduction. From potentials of -0.65 V to the potential coincident with the first phase transition, the permeability of the layer to these metal ions increases with an increase in the applied negative potential. The second phase transition represents a process involving the growth and coalescence of defects.