The comparison of the miscibility behavior in binary monolayers of type phosphatidylcholine/phosphatidylethanolamine [system 1, dilauroylphosphatidylcholine/dilauroylphosphatidylethanolamine; system 2, dilauroylphosphatidylcholine/dimyristoylethanolamine; system 3, dilauroylphosphatidylcholine/dipalmitoylphosphatidylethanolamine] leads to the essential results that both the differences in the number of CH2 units of the hydrocarbon chains and the film state (liquid-expanded film M(e) or condensed film M(c)) in the monolayers are responsible for miscibility or immiscibility of the components in the mixed films. The investigations of the binary monolayers were carried out by application of a variety of surface and spreading techniques (mixed spreading, separated spreading, spreading pressure, compression isotherms, long-time investigations). From these studies of the compression and spreading properties of the above mentioned three binary systems and the application of the surface-phase rule we obtained the phase diagrams of the mixed monolayers, In system 1 we found that both components are completely miscible in the film state M(e) and only partially miscible in the film state M(c). The same situation characterizes system 2, In the mixed monolayers of system 3 we found only partial miscibility of the phospholipids.