We demonstrate that the surface potential of charged liposomes can be determined by second harmonic generation. The Gouy-Chapman model is used to describe the electric double layer. From the experimental data, we obtained the surface charge density and the surface potential of liposomes consisting of the negatively charged phospholipid dioleoylphosphatidylglycerol. The surface potential was found to be in the range of 20-100 mV depending on the concentration and valence of the electrolyte (NaCl or MgSO4) in solution. The charge density was found to be similar to1.3 x 10(14) per cm(2), corresponding to similar to 70 Angstrom (2)/charge, which is comparable to the area per phospholipid headgroup,