A detailed miscibility analysis of the three systems of binary phospholipid monolayers (system dimyristoylphosphatidylethanolamine/dipalmitoylphosphatidylethanolamine, system dipalmitoylphosphatidylethanolamine/distearoylphosphatidylethanolamine and system dilauroylphosphatidylethanolamine/distearoylphosphatidylethanolamine) was carried out by application of a variety of spreading and surface techniques to produce mixed monolayers in different mixing states. Studies of the compression and spreading behavior of these binary systems and the application of the surface-phase rule form a basis for the determination of binary phase diagrams. From these phase diagrams of the binary monolayers we can analyze and compare the miscibility properties in the mixed films. We found complete or partial miscibility in the binary monolayers in dependence on the film state (liquid-expanded or condensed films) and the chain length differences between the mixing components.