The reactions of Ca(NO 3 ) 2 + Na 3 PO 4 in water and water/sodium bis(2-ethylhexyl) sulfosuccinate (AOT)/hydrocarbon microemulsions saturated CO 2 with various molar ratios of water to surfactant R, oil phases, and surfactant concentrations were investigated by isothermal titration calorimetry. The product of the reaction was confirmed to be sodium-and-carbonatesubstituted hydroxyapatite (NaCO 3 HAP) by Fourier transform infrared spectra (FTIR), energy dispersive spectrometry (EDS), and X-ray diffraction (XRD). From calorimetric measurements, the molar enthalpies of solution of water in the AOT/n-dodecane system, and the molar enthalpies, the rate constants, and the activation energies of the reactions were determined. It was found that the enthalpy of solution of water in AOT/n-dodecane micells and the molar enthalpy of the reaction in the microemulsions increased with the decreases of R until R = 7; below that they kept almost constant. It may be attributed to the increase of the ratio of the bound water to the free water with the decrease of R until there was no free water when R < 7. However, the reaction rate constant k 1 was affected by the ionic strength of the medium and log k 1 showed a linear dependence on 1/R in the whole range of R we investigated. It was also observed that the rate constant and the enthalpy of the reaction remained almost unchanged when the surfactant concentration and the nature of oil phase varied. C