AbstractPartial melting of tonalitic gneisses in the 2.7 Ga Badcallian granulite facies metamorphic episode in the Scourian complex of north‐west Scotland produced a suite of granitic to trondhjemitic liquids. On cooling and excavation of the complex, these melts underwent fractional crystallization and the residual liquids eventually became water saturated. Comparison with experimental data suggests that water saturation would have occurred in these melts at around 620–700°C. From the retrogradeP–T‐time path followed by the complex it is estimated that H2O‐dominated fluids were exsolved from these melts atc.2.5 Ga. It is proposed that these fluids were the cause of the 2.5 Ga Inverian retrogression of the Scourian complex and that water‐saturated melts formed during the crystallization of the leucogneisses were intruded as a suite of pegmatites. The timing of pegmatite intrusion is consistent with this proposition as are the temperature estimates, timing, distribution and nature of the Inverian phase of metamorphism. It is likely that the crystallization of melts is an important process in bringing about hydrous retrogressive metamorphic episodes in a number of other basement terrains, such as West Greenland a