Synthetic colloidal calcium hydroxyapatite (Ca(10)(PO(4))(6)(OH)(2): CaHap) was treated with pyrophosphoric acid (H(4)P(2)O(7): PP) in acetone and the materials were characterized by XRD, TEM, FTIR, and N(2) and H(2)O adsorption measurements. XRD patterns and morphology of CaHap particles were essentially not changed by the modification. The additional amount of PO(4) of CaHap was increased with an increase of PP concentration and the Ca/P molar ratio of the particles decreased from 1.62 to 0.81. IR results indicated that the isolated surface POH band developed with increasing the PP concentration up to 6.0 mmol dm(-3) by the reaction of isolated surface POH groups of CaHap and pyrophosphoric acids. Above 10.2 mmol dm(-3), a hydrogen-bonding surface POH band appeared at 2913 cm(-1) and enlarged with increasing the PP concentration, while the isolated surface POH band was weakened. The results of N(2) and H(2)O adsorption measurements revealed that the modified particles aggregated compared to the unmodified ones, which would be due to the formation of hydrogen-bonding surface POH groups among the particles.