A novel method to measure the surface charge density at a hanging mercury drop electrode (HMDE) coated with a self-assembled phospholipid monolayer mimicking a biological membrane is described. The charge density is obtained by analogical integration of the capacitive current which flows at constant applied potential as a consequence of a slight contraction of the mercury drop. The contraction must be carried out while keeping the neck of the lipid-coated mercury drop in contact with the lipid film spread on the surface of the electrolytic solution. The validity of this procedure is tested by comparison with differential capacity measurements carried out at a lipid-coated HMDE, fully immersed into the solution, under otherwise identical conditions. The potential of this method in the investigation of the properties of self-assembled lipid monolayers, either pure or incorporating lipophilic species, is briefly outlined. The method is applied to the determination of the charge density of tetraphenylphosphonium cations adsorbed in the polar head region of a phosphatidylserine monolayer supported by the HMDE.