The miscibility of dimyristoylphosphatidylcholine (DMPC) and poly(ethylene oxide) lipids (PEG lipids) in monolayers has been assessed through surface pressure and surface potential measurements. These experiments performed under equilibrium conditions at constant area revealed a strong dependence of the interfacial behavior of these mixtures on the polymer chain length. While for the lipid with the short PEO chain composed of 5 monomer units the mixtures with DMPC displayed almost an ideal behavior, for longer PEO chains (13 and 31 monomer units) pi-delta isotherms considerably deviated from ideality. The effect of hydrocarbon and polymer chain orientation and of the structuring of interfacial water induced by PEO lipids on their mixing behavior with DMPC and on calculated effective dipole moments is discussed and analyzed.