Observations from ultrahigh-pressure metamorphic rocks combined with experimentally determined phase relations provide a framework for understanding fluid-mediated mass transfer in deeply subducted continental crust. At temperatures below 650 °C, aqueous fluids derived from dehydration reactions involving hydrous phases contain limited amounts of solutes. At temperatures of 700–800 °C, a supercritical fluid with a composition intermediate between aqueous fluid and hydrous melt might be present. The most significant mass transfer at ultrahigh-pressure conditions occurs at 800–1000 °C, where subducted crust undergoes partial melting related to the breakdown of the hydrous mineral phengite. Partial melting leads to a significant change in the composition and density of the rocks, and also affects the rheology of deeply subducted crust.