Conformational changes of membrane proteins are accompanied by deformation in the surrounding lipid bilayer. To gain insight into the energetics of membrane deformation, the phase behavior of dimyristoylphosphatidylcholine (DMPC) membranes in the presence of dipole potential, ψd, modifiers were investigated by differential scanning calo-rimetry. 7-Ketocholesterol, which weakens ψd and reduces membrane-perpendicular dipole-dipole repulsion, causes a discrete second peak on the high temperature side of the main transition, whereas 6-ketocholestanol, which strength-ens ψd and increases membrane-perpendicular dipole-dipole repulsion merely produces a shoulder. Measurements on pure DMPC vesicles showed that the observed temperature profile could not be explained by a single endothermic pro-cess, i.e. breaking of van der Waals forces between hydrocarbon chains alone. Removal of NaCl from the buffer caused an increase in the main transition temperature and the appearance of an obvious shoulder, implicating polar interac-tions. Consideration of the phosphatidylcholine headgroup dipole moment indicates direct interactions between phosphatidylcholine dipoles are unlikely to account for the additional process. It seems more likely the breaking of an in-plane hydrogen-bonded network consisting of hydrating water dipoles together with zwitterionic lipid headgroups is responsible. The evidence presented supports the idea that the breaking of van der Waals forces between lipid tails required for the main phase transition of PC membranes is coupled to partial breaking of a hydrogen-bonded network at the membrane surface.