For aluminium oxide-hydroxyapatite composites sintered in air, decomposition of hydroxyapatite to tricalcium phosphate was initiated at temperatures below 1000 degreesC and completed at 1200 degreesC. Composites of aluminium oxide and various apatites (hydroxyapatite, fluorapatite and chlorapatite) as well as the pure materials were hot isostatically pressed at 1200 degreesC for 1 h at a pressure of 160 MPa. Due to the closed system used, the thermal stability was improved and no phase changes were detected in the hot isostatically pressed aluminium oxide-apatite composites. These materials were crushed and thereafter heat-treated in air at several temperatures in order to study the phase stability. In the fluorapatite and chlorapatite based composites, the apatite reacted with the moisture in the air and partly converted to oxyhydroxyapatite and decomposition of the latter phase was initiated. The phase composition in the materials was evaluated by X-ray powder diffraction and the microstructure of the hot isostatically pressed samples was studied in a scanning electron microscope.