The reaction of biological and synthetic hydroxyapatite Ca5(PO4)3OH (HAP) with Zn2+ ions is investigated as a function of Zn2+/Ca2+ molar ratio, time, temperature and electrolyte type (NaCl, NaHCO3, Na2HPO4) by means of pH, pZn, pCa measurements, in aqueous solution. Biological powdered HAP invariably affords an almost quantitative reaction, while Zn2+ precipitated only partially by reaction with cubelets of biological HAP. Using powdered biological HAP and synthetic HAP (dried at 100 degrees C), the reaction with Zn2+ ion is fast and takes place without addition of precipitating anion; synthetic HAP (dried at 1000 degrees C) reacts if free phosphate ions are present. The solid phases separated after different reaction times are investigated by means of X-ray diffraction (XRD), IR, SEM techniques and elemental analysis (C,H,N). The solid phases contain Zn3(PO4)2.4H2O (Hopeite) at the beginning of reaction and CaZn2(PO4)2.2H2O (Scholzite) at the equilibrium.