Summary: Phase equilibria involving solids, liquids, and vapours on the join Ca(OH)2-CaF2-Ca3(PO4)2-H2O were determined at 1000 bars. The temperatures and liquid compositions (weight %) of the isobaric invariant reactions encountered were: portlandite+fluorite → liquid, 687° C, 70 % Ca(OH)2 30 % CaF2; portlandite+fluorite+vapour → liquid, 675° C, 69% Ca(OH)2 26 % CaF2 5 % H2O; portlandite+fluorite+apatite → liquid, 675°C 73 % Ca(OH)2 24 % CaF2 3 % Ca3(PO4)2; portlandite+fluorite+apatite+vapour → liquid, 665° C, 70 % Ca(OH)2 22 % CaF2 3 % Ca3(PO4)2, 5 % H2O. The temperatures of these reactions were determined from over 400 experimental runs at selected pressures from 250 to 4000 bars. The composition of the apatite involved in the last two reactions was 60 % hydroxyapatite 40 % fluorapatite and in subsolidus regions in the presence of water, fluorapatite did not crystallize but fluorhydroxyapatite solid solutions coexisted with vapours containing up to 2·5 % hydrogen fuoride. These observations suggest that hydroxyapatite is more stable than fluorapatite at elevated temperatures and pressures and would be the composition expected to crystallize under igneous and metamorphic conditions.