Under hydrous conditions the stability field of the assemblage Mg-cordierite+K feldspar+quartz is limited on its low-temperature side by the breakdown of cordierite+K feldspar into muscovite, phlogopite and quartz, whereas the high-temperature limit is given by eutectic melting. The compatibility field of the assemblage ranges from 530° C to 745° C at 1 kbar \(P_{{\text{H}}_{\text{2}} {\text{O}}}\), from 635 to 725° C at 3 kbars \(P_{{\text{H}}_{\text{2}} {\text{O}}}\), from 695 to 725° C at 5 kbars \(P_{{\text{H}}_{\text{2}} {\text{O}}}\) and terminates at 5.5 kbars \(P_{{\text{H}}_{\text{2}} {\text{O}}}\). Most components not considered in the model system will tend to restrict this field even more. However, the condition \(P_{{\text{H}}_{\text{2}} {\text{O}}}\) < Ptotal will increase the range of stable coexistence drastically, making the assemblage common at elevated temperatures from contact metamorphic rocks up to intermediate pressure granulites of appropriate bulk composition.