X-ray diffraction has been used to determine the phase behavior and the surface forces between bilayers containing lipids with covalently attached polyethylene glycol (PEG-lipids). To examine the effects of the polymer and lipid components, we studied PEG-lipid/phospholipid mixtures in which we varied the molecular weight of the polymer (350-5000 Da) and the matrix phospholipid composition (different hydrocarbon chains and various cholesterol concentrations). Depending on the concentration of the PEG-lipid, the molecular weight of the polymers, and the lipid composition, the molecules self-assembled into various amphipathic structures-bilayers, bilayers with interdigitated hydrocarbon chains, and micelles. The polymer extended away from the lipid-polymer interface and formed a steric repulsive barrier. Using an osmotic stress technique, we measured the extension of the polymer from the bilayer surface, and the pressures required to compress the PEG between apposed bilayers. The steric repulsive pressures were modeled reasonably well with mean-field and scaling theories of polymer behavior when polymer polydispersity was taken into account. The extension and magnitude of this barrier depended on the concentration of PEG-lipid in the bilayer and was relatively insensitive to lipid composition: 10 mol% PEG-2000 incorporated into gel, liquid crystalline, or cholesterol-containing bilayers generated a barrier that extended about 65 A away from each bilayer surface. The maximum concentration of PEG-lipid that could be incorporated into the bilayer, however, depended on the lipid composition and decreased with either the incorporation of lysolipids or with decreasing the phospholipid acyl chain length. These studies demonstrated that the polymer and lipid influenced each other to determine the phase behavior and intersurface forces of this hybrid system.