Composites of hydroxylapatite ( HA) with pure zirconia, and 3 and 8% Y2O3 in zirconia, were pressure-less sintered at temperatures from 900 to 1300 degrees C, and hot-pressed at 1200 degrees C in argon gas atmosphere for 1 h. The reactions and transformations of phases were monitored with X-ray diffraction and thermal analysis. At sintering temperatures higher than 1000 degrees C, calcium from HA diffused into the zirconia phase, and the HA phase decomposed to tri-calcium phosphate (TCP). Above about 1200 degrees C, CaZrO3 was formed. These reactions and transformations were interpreted in terms of the ZrO2-CaO phase diagram. On the other hand, zirconia and hydroxylapatite phases in hot pressed composite remained mainly stable suggesting that air in the sintering environment increased the reactivity between the phases. The highest densification was found in a composite initially containing 10% monoclinic ZrO2 sintered at 1300 degrees C. The densification of the composites decreased at lower sintering temperatures and higher zirconia contents upon air-sintering. (C) 2005 Springer Science + Business Media, Inc.