Sintering of two hydroxyapatite (HA) samples with different Ca/P ratios was studied as a function of thermal pretreatments, sintering temperature and additives (0-0.6 ion % Li+ or 0-5 ion % Mg2+). The samples were sintered in air and characterized by density measurements, scanning electron microscopy, differential thermal analysis, X-ray diffraction and dilatometry. Upon sintering, samples with Ca/P ratio of 1.51 (HA C) transformed to beta-Ca3(PO4)2 and Ca10(PO4)6(OH)2, resulting in materials with low densities and containing agglomerated beta-Ca3(PO4)2 when sintered above 1200 degrees C. Samples with a Ca/P ratio of 1.77 (HA S), without beta-Ca3(PO4)2, showed better sinterability and homogeneous microstructures. Li+ additions favoured liquied-phase sintering and reduced the beta-Ca3(PO4)2 content in sintered materials. Mg2+ additions did not result in higher densities, but inhibited the hydroxyapatite grain growth rate. A significant percentage of the added Mg2+ was incorporated into the beta-Ca3(PO4)2 structure.