Layered calcium octyl phosphate (CH3(CH2)(7)OPO(3)Ca(.)1.6H(2)O: CaOP), which is composed of a multilayer alternating bilayer of octyl phosphates and a dicalcium phosphate dihydrate (DCPD)-like phase, was thermally treated in vacuo and the intercalation of n-alkyltrimethylammonium ions into the materials was examined. The octyl groups in the layer were eliminated by outgassing above 250degreesC to give the amorphous calcium phosphates. Further, the specific surface area was steeply increased and mesopores with a diameter of ca. 2.0 nm were formed. IR results indicated that the surface P-OH groups were generated by outgassing at 250degreesC. When the CaOP outgassed at 250degreesC was treated with n-alkyltrimethylammonium ion solutions (carbon number of alkyl group, n = 14-18), three XRD peaks reappeared below 2theta = 15degrees and the d-spacing ratio of these peaks was 1:1/2:1/3. These facts indicate that the n-alkyltrimethylammonium ions were intercalated into the amorphous calcium phosphate phases. (C) 2004 Elsevier Inc. All rights reserved.