Synthetic colloidal calcium hydroxyapatites (Ca-10(PO4)(6)(OH)(2), CaHAP), treated with hexyl, octyl, and decyl phosphates in acetone-water solutions, were characterized by various means. XRD patterns of the modified materials showed a strong peak and two weak ones besides the peaks due to CaHAP. The d value of these three additional peaks linearly increased with an increase in the carbon number of the phosphates. Upon increasing the concentration of the phosphates in the treating solution, these peaks grew, while the peaks due to CaHAP diminished. After the treatment, only rod-shaped particles were observed, testifying that the surfaces of the original CaHAP particles were modified without formation of new particles. These results show that the modified CaHAP particles have a surface-layered phase consisting of multilayers of alternating octacalcium phosphate (OCP)-like phase and bimolecular layer of alkyl groups. Alkyl groups and H2O in the layers were removed by outgassing above 200 degrees C to give the materials having mesopores.