Recent water-undersaturated phase equilibrium data on the subsystems of the granite–H 2 O system have provided important new constraints on the topology of the cotectic surfaces and hence on the compositional evolution of felsic magmas. The effect of water on phase relations can be deduced from a comparison of anhydrous and H 2 O-saturated data or from data obtained in the presence of a CO 2 -bearing fluid. However, although new experimental evidence indicates that the silica enrichment of evolving H 2 O-undersaturated, H 2 O-unbuffered melts during the co-precipitation of quartz and feldspar is as previously thought for orthoclase-rich compositions, it suggests that such a trend is considerably less for Ab-rich compositions. For water-poor trachytic melts, the newly recognised strong destabilisation of the sanidine melt component relative to the anorthite melt component with increasing water content indicates that the co-precipitation of two feldspars will result in saturation of the melt with ternary alkali feldspar at an earlier stage (i.e. higher melt anorthite content) than previously thought. This, in turn, implies that the melt differentiation path will have a greater component of anorthite depletion during the equilibrium co-precipitation of ternary feldspars and that the melt will remain in the peritectic region of the two feldspar plus liquid surface over a greater interval of crystallisation, thereby enhancing the possibility that the resoption of plagioclase during the early stages of equilibrium with alkali feldspar may go to completion. Comparison of CO 2 -free and CO 2 -bearing haplogranitic phase equilibrium data suggests that CO 2 may be playing an independent part in the modification of phase equilibria and may induce a significant destabilisation of the orthoclase melt component.