Surface pressures and surface potentials have been obtained as a function of surface area/molecule for monomolecular films of 1,2-dimyristin, 1,2-dimyristoyl-3-cephalin (L-configuration_ at the air-water interface. The studies were carried out in the temperature range +1° to +37°C. The results obtained are interpreted in terms of electrostatic repulsions between the polar groups increasing in the order dimyristin