Externally applied electric field gradients gave rise to lateral concentration gradients in monolayers of certain binary lipid mixtures. For binary mixtures of dihydrocholesterol and dimyristoylphosphatidylcholine, the application of an electric field gradient at pressures below the critical pressure produced a liquid-liquid phase separation in a monolayer that is otherwise homogenous. At pressures slightly above the critical pressure, a field gradient produced a large concentration gradient without phase separation. The lipid concentration gradients can be described by equilibrium thermodynamic chemical potentials. The observed effects appear to be relevant to the structure and composition of biological membranes.