Composites of 25 wt.% monoclinic zirconia and 75 wt.% hydroxyapatite (HA) (with and without 5 wt.% ZrF4) were hot pressed to investigate their mechanical properties and phase stability. HA decomposed to tricalcium. phosphate when there was no ZrF4 present. Addition of ZrF4 into the composite improved its thermal stability and mechanical properties. Substitution of OH- by F- ions was verified by the change in HA's hexagonal lattice parameters. A fracture toughness of 2.1 MPa root m and microhardness of 6.9 GPa were calculated for the composite containing ZrF4. (c) 2006 Published by Elsevier Ltd. on behalf of Acta Materialia, Inc.