Synthetic hydroxyapatites (HA) with widely different crystallinity degrees were prepared starting from calcium hydroxide and phosphoric acid. The powders were carefully characterised by various physico-chemical techniques. Additions of Ca(OH)(2) were performed in order to improve HA thermal stability to firing treatments up to temperatures significantly higher than those commonly employed for safe HA sintering. A positive effect in this sense was found. yielding HA almost purr samples up to 1400-1450 degrees C and maintaining considerable HA concentrations up to 1600 degrees C. An explanation in terms of stoichiometric defectiveness, linked to the quantitatively estimated contemporary presence of HPO42- and CO32- substituting PO43-and OH-, was attempted.