The work is aimed to understand the effect of nonionic surfactants on the stability of lipid membranes and to characterize the aggregate structure on a molecular level. We studied the lyotropic phase behavior of mixtures of the lipid 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) with nonionic surfactants of the homologous series C12En (n = 2, 4, 6) as a function of composition (up to a mole ratio detergent-to-lipid, R-D/L = 1), temperature (0 degreesC < T < 50 degreesC), and relative humidity (3% < RH < 100%). The latter parameter was used to adjust and to vary the hydration degree of the samples. The results were summarized in terms of RH-T phase diagrams. At full hydration (RH = 100%), all systems formed mixed bilayers. The studied nonionic detergents promoted the formation of the inverse hexagonal phase, Hit, at reduced hydration and above a critical temperature near 20 degreesC, i.e., in the left upper corner of the phase diagram. Solid phases were observed at lower temperatures. C12E2 showed some special properties. This detergent with a short ethylene oxide (EO) chain stabilized the solid phase at low concentrations, and it extended the range of the Hit phase to higher RH as compared with the effect of the detergents with longer EO chains. The different phases were characterized in terms of molecular order and conformational properties of the EO chains. In membranes, they existed in a more ordered state than in assemblies of the pure detergent at equal external conditions. In a methodical respect, this work demonstrates that hydration-tuning infrared linear dichroism spectroscopy is well-suited to detect phase transitions of lipid/detergent mixtures between fluid and solid structures and between aggregates of lamellar and nonlamellar morphologies.