The hydration of two peculiar unsaturated lipids with phosphatidylethanolamine (PE) and phosphatidylcholine (PC) headgroups, namely dioctadecadienoyl PC (DODPC) and dioleoyl PE (DOPE), has been studied at room temperature by infrared (IR) spectroscopy including the linear dichroism. Oriented films of these lipids subjected to an atmosphere with definite values of relative humidity were investigated. The choice of DODPC and DOPE was motivated by their common property of undergoing a lyotropic phase transition, which, in each case, nearly exclusively affects the polar part of the lipid aggregates and is, thus, termed "solvation-induced" transition. The temperature-RH phase diagrams of both lipids were determined. The headgroups of both lipids form quasi-crystalline structures at small RH. The long axes of the PE and PC moieties orient essentially parallel with respect to the polar interface of; the lipid aggregates. Model-based analyses of IR linear-dichroism data are consistent with the idea that about 8-12 DOPE molecules with disordered chains assemble within one cross-section layer of a ribbonlike phase, P-alpha. Dry DODPC forms a subgel phase (SG(1)) with all-trans acyl chains. At a critical water activity near 0.6, the molecular disorder in the polar region of the lipid aggregates increases distinctly. This "headgroup melting" reflects the solvation-induced transitions leading to the SG(II) and H-II phases in DODPC and DOPE, respectively. Hydrogen-bonding specifics due to the polar parts of both lipids were probed by means of H --> D exchange. Particularly stable H bonds are indicated to exist in dried DOPE.