  The hydration process 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 was found to proceed in two
stages. In the early stage, until about 100 s after hydration, lipid bilayers
on a solid substrate swell about 12 $\rm{\AA}$ and reach a quasi-stable state.
In the late stage, after several hundreds of seconds, lipid bilayers gradually
peel off from the stack. On the other hand, in the case of a lipid in the gel
phase, only the early stage is observed. These behaviors correspond to the
capacity for the giant vesicle formation depending on the lipid phases. The
kinetics of the peeling-off process is discussed in terms of Kramers' formulas.
