Phase properties of substrate-supported nanotubular dimyristoylphosphatidylcholine ( DMPC) bilayers confined within nanoporous channels of anodic aluminum oxide were characterized by DSC and compared with unsupported vesicles. In addition to the main phase transition, all samples exhibited a pretransition with a characteristic midpoint hysteresis between heating and cooling scans. The pretransition indicates that nanotubular bilayers could exist in a ripple phase, whereas hysteresis points to a similarity in the phase transition mechanisms. Observance of the ripple phase in lipid nanotubes is an indication of fully hydrated and only slightly perturbed bilayer surface.