High-grade metapelitic gneisses rich in cordierite and K-feldspar at Cooma, N.S.W., Australia, show microstructural evidence of partial to complete replacement of cordierite grains with biotite inclusions by fine-grained, symplectic intergrowths of andalusite, biotite and quartz. The replacement occurred during the later stages of the history of the Cooma Complex, under retrograde conditions. An equation for the local reaction: cordierite+red-brown biotite+H2O+K+ ⇆ andalusite+green-brown biotite+quartz+H+ can be balanced approximately, using compositions obtained from microprobe analyses of the minerals involved. The volumes occupied by the solid reactants and solid products are nearly identical. The potassium and water were derived from an unknown external source, possibly connected with the replacement of K-feldspar by muscovite in nearby rocks.