An experimental investigation is described oil the variables that affect the lateral diffusion coefficient (D-lat) of dimyristoylphosphatidylcholine, a zwitterionic phospholipid, and the effective charge (Z(ef)) oil liposomes. The lateral diffusion coefficient was obtained from the dielectric relaxation time of the zwitterionic phospholipids in the bilayer, and the effective charge oil the external monolayer was estimated from microelectrophoretic tic mobility measurements by means of the Henry and Coulomb equations. The measurements were performed at different pH values and salt (KBr) concentrations as well as in two physical states of the phospholipid: the liquid-crystalline phase and gel phase. The Z(ef) and D-lat Values in the gel phase are always lower than those in the fluid phase. A very small change of pH (similar to 0.5 pH units) caused a pronounced variation of the effective charge and the lateral diffusion coefficient. Both variations are correlated, which demonstrates that the adsorption of the ions that determine the electrokinetic potential also controls the lateral diffusion of dipolar phospholipids in the bilayer and the effective charge on the external surface of the liposomes.