In this study we investigated the origin and the magnitude of surface potentials of monolayers composed of lipids to which polyethylene glycol(PEG) is covalently bonded. Under the first set of experimental conditions we measured the surface potential of lipid monolayers composed of different lipids. In the second set we added free PEG to the subphase of the monolayers. In the third series we investigated the variation of the polyethylene glycol moieties covalently attached to the lipids. The experimental data allowed the separation of different contributions to the surface potential originating from the hydrocarbon side chain, the surface charge, and PEG. For lipid monolayers in the gas phase up to a surface pressure of 10 mN/m, the contribution of the PEG dominated the surface potential and the surface pressure while near the collapse point the lipids gave the main contribution to both parameters. The results showed a good correlation to recent measurements of the surface potentials of liposomes formed from the same lipids.