The surface of synthetic colloidal calcium hydroxyapatite (Ca-10(PO4)(6)(OH)(2); CaHap) was treated with pyrophosphoric acid (H4P2O7; PP) in acetone and the materials were characterized by XRD, thermal analysis, N-2 adsorption, TEM, and FTIR. No remarkable change in XRD patterns or in particle morphology by the modification was observed. The additional amount of PO4 of CaHap was increased with an increase in PP concentration. The Ca/P molar ratio of CaHap was decreased from 1.62 to 1.35 by the modification. IR results indicated that the PP reacts with surface P-OH groups of CaHap to form additional surface P-OH groups as follows: surface P-OH + H4P2O7 --> surface P-O-PO(OH)(2) + H3PO4. When the modified CaHap with Ca/P molar ratio less than 1.50 was treated at 850 degreesC in air, the materials transformed into beta-Ca3(PO4)(2). (C) 2003 Elsevier Inc. All rights reserved.