Calcium hydroxyapatite (CaHAP) particles synthesized by a wet method have been modified by treating them with water–acetone solutions of ethyl phosphates (EP). The modified CaHAP particles were characterized by various means. XRD patterns of the modified CaHAP particles showed a strong peak at d= 1.40 nm in addition to the peaks characteristic of CaHAP. With increasing EP concentration the XRD peaks of CaHAP were weakened while the new strong peak grew rapidly. The original particles were modified by EP, with no formation of new particles. These results indicate that treatment with EP induces the formation of a layered structure in the surface layer of the particles. The H₂O and C₂H₅ groups present in the modified particles were removed by heating above 350 °C in air, which resulted in the immediate disappearance of the XRD peaks due to the layered structure, so that only the CaHAP peaks were observed. Outgassing of the modified materials above 300 °C gave mesoporous CaHAP particles.