We studied the coupling of a polymeric amphiphile undergoing a transition from an expanded to a collapsed state of the hydrophilic polymer chain upon heating above 29°C with free and solid supported lipid bilayers. Lateral diffusion measurements and studies of vesicle shape-changes confirmed previous calorimetric studies, suggesting that the amphiphile remains incorporated in the giant vesicles and the supported bilayer during the transition. The two-dimensional hydrodynamic radii of the amphiphile in the expanded and collapsed state was measured by making use of the strong quadratic dependence of the diffusion coefficient on the radius of the diffusant ( D ∞ r −2) due to the frictional coupling between the bilayer and the supporting substrate. The hydrodynamic radius changed by a factor of 3 during, the transition from the expanded to the collapsed state. Shape-changes of multilamellar and thin-walled vesicles during the transition were observed by phase contrast microscopy. In the expanded state of the polymer, the vesicles exhibit a smooth contour, although non-trivial and non-symmetric shapes, were found. Heating over the collapse transition induces pronounced budding of the thin-walled vesicles, while the multilamellar liposomes exhibit only a transient formation of long wavelength protrusions. The shape-changes are explained in terms of the coupling between phase separation and induced curvature.