The hydration of multi-stacked phospholipid bilayers to form giant vesicles was investigated by time-resolved small-angle X-ray scattering. The hydration of lipids in the liquid-crystalline phase proceeds in three stages. In the early stage, water molecules penetrate between lipid bilayers, and in the intermediate stage the lipid bilayers swell and reach a quasi-stable state. In the late stage, lipid bilayers gradually peel off from the stack. On the other hand, in the case of the gel phase, the peeling-off stage is not observed. These behaviors correspond to the capacity for giant vesicle formation depending on the phases of lipid. The kinetics of the peeling-off is discussed in terms of Kramers' formulas. (c) 2008 Elsevier B. V. All rights reserved.