Fast lateral proton conduction along the lipid/water interface has recently been experimentally demonstrated in our laboratory [Teissié, J., Prats, M., Soucaille, P. & Tocanne, J.F. (1985) Proc. Natl Acad. Sci. USA, in the press]. The present study gives a more precise description of the way various physical parameters can affect this process. The dependence of the distance covered by the proton on time is demonstrated to be quadratic. Increasing the speed of stirring in the injection compartment or the amount of injected acid or the contact between the monolayer and the acidic subphase increased the efficiency of the proton transfer. Raising the strength of the buffer in the bulk phase inhibited proton conduction. Results from experiments where the transfer of protons from the bulk phase to the interface was modified, suggested the occurrence of an 'energy barrier' limiting the access of protons from the bulk phase to the lipid polar head region.