The surface charge density of lipid monolayers is the function of the ionization degree of their molecules. In the case of monolayers formed from fatty acids, the surface charge density increases on alkaline subphases and decreases on acidic subphases. In studying ionized monolayers one needs to know how pH and dissolved salts of subphase affect the ionization degree of monolayer molecules. From this point of view the properties of monolayers formed from stearic acid and zwitterionic phospholipids were considered as pH-dependent. The boundary potential jump in any amphiphilic monolayer includes dipole and surface constituents. Within the pH interval from 1 to 5 all isochors of surface pressure are merged and exhibit the horizontal shape. Phosphatidylcholine monolayers are widely used in studies and reconstitution of various biophysical processes in biological membranes. Ordered oriented hydrocarbon radicals of phosphatidylcholine molecules simulate a hydrophobic zone of biological membrane while the polar heads simulate their external hydrophilic surface.