The aim of this study was to examine the electrokinetic properties of liposomes containing either phosphatidylethanolamine (PE) or phosphatidylserine (PS). Zeta potential was determined by laser-Doppler microelectrophoresis. The presence of PE enhanced the slight negative charge on the phosphate group of phosphatidylcholine. A linear relation between zeta potential and PE concentration was found. Absolute values of zeta potential were higher in PS liposomes and showed an exponential dependence on the PS content. The electrophoretic mobility depended not only on the percentage of aminophospholipid but also on the size of vesicles. This behavior can be explained by the relaxation effect. From geometrical considerations, it was possible to calculate the theoretical surface charge densities. Comparing experimental with theoretical values, an underestimation of zeta potential was found by the electrokinetic classic theory in the experimental conditions used.