The addition of 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) to aqueous phospholipid membranes leads to perturbation of the bilayer. In the case of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), calorimetric and small-angle X-ray scattering analyses indicate that effects are already apparent at bound molar HFIP/lipid ratios of less than 1:150, with a pronounced decrease in the temperature of the main (gel to liquid crystalline) phase transition and a decrease in the intensity of the first- and second-order scattering reflections. As the HFIP concentration is raised further, at bound molar HFIP/lipid ratios > 2:1, uniform isotropic particulate structures are formed with higher intrinsic curvature than the parent liposomes. These observations are supported by the results of thin-film experiments and are consistent with the formation of DMPC/HFIP adducts that are detergent-like in nature. In the case of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) the effects are much less marked, with no blebbing observed over a comparable range of HFIP concentrations. Although HFIP interacts strongly with DOPC membranes, it appears that membrane rupture is not promoted as readily with this lipid. Data from electron microscopy, laser correlation spectroscopy, and marker release experiments suggest that some of the immediate (nonequilibrium) effects of HFIP on membranes are the consequence of microinhomogeneity in water/HFIP mixtures. On the basis of our observations, we propose a model for the interaction of HFIP with phospholipid membranes.