The responses of olivine-spinel, spinel--mixed oxide, and basalt-eclogite phase transitions in the mantle to the pressure change resulting from deglaciation are evaluated by using the analysis of O'Connell and Wasserburg. For all cases, movement of the phase boundaries can at best account for only a small fraction of the observed uplift. Thus plastic flow of the mantle is the most likely mechanism for postglacial rebound.