Synchrotron X-ray reflectivity and grazing incidence diffraction were applied to study the structures of phospholipid Langmuir-Blodgett films under controlled humidity. Distearoylphosphatidylethanolamine (DSPE) and DSPE-PEO lipopolymers, in which a poly(ethylene oxide) chain with n = 8, 17, or 45 EO units is covalently linked to the polar headgroup of DSPE, were used. When the relative humidity was changed from below 2 to 97%, the phosphate headgroup in a DSPE film was hydrated with a concomitant thickness increase of 4.2 A and an electron density decrease. The swelling of the monolayer was found to be a reversible process without any significant roughening of the film. Subtle differences in the film thickness could be detected for DSPE monolayers transferred under various lateral pressures. The degree of lateral ordering of the alkyl chains in DSPE monolayers increased considerably when the humidity was raised. In the case of DSPE-PEO with eight EO units, the hydration of the monolayer was also found to be reversible, but the water uptake was larger due to the presence of the hydrophilic polymer interlayer, which is located between the substrate and the DSPE moieties. Under high humidity, the lipopolymer monolayer with n = 17 exhibited a well-defined, layered structure similar to that of DSPE-PEO with n = 8. In the case of n = 45, however, there are indications for a significant intermixing of the polymer with the DSPE moieties, probably resulting in the formation of hydrophobic nanodomains of closely packed alkyl chains that are immersed into the hydrated polymer.