Microstructural evidence indicates that fibrolitic sillimanite partly replaced of the highest-grade metamorphic assemblage in the high-grade metapelitic gneisses, migmatites and granodiorite of the Cooma Complex, southeastern Australia. The sillimanite is interpreted as having formed by the action of mobile hydrogen ions on pre-existing minerals, base cations being released and removed from the sites of reaction. The hydrogen ions may have been liberated during widespread hydration of cordierite. The general base-leaching process may be applicable to the formation of late sillimanite in high-grade, migmatitic gneisses in other areas.