A novel procedure for the measurement of the absolute value of the surface dipole potential chi of self-assembled lipid monolayers, which makes use of a phospholipid monolayer supported on mercury, is described. It consists of increasing the tilt angle of the lipid molecules with respect to the monolayer normal by expanding progressively the supporting mercury drop and in measuring the charge on the mercury surface that accompanies the drop expansion. Dipole potential values of +145 +/- 10 mV for neutral dioleoylphosphatidylcoline (DOPC) and dioleoylphosphatidylserine (DOPS) and off +30 +/- 3 mV for neutral dioleoylphosphatidic acid (DOPA) were determined. A gradual increase in the negative charge of the DOPS and DOPA headgroups due to an increase in pH causes an increase in chi. This suggests that the dipole potential of about +145 mV in neutral dioleoylphospholipids stems from the ester linkages to the glycerol backbone (which in DOPA monolayers are screened by the water molecules) and possibly, to a minor extent, from the orientation of the hydration water molecules. The latter contribution is small in neutral phospholipid monolayers, but becomes progressively more positive with an increase in the negative charge on the polar heads of the lipids.