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Abstract:The setting reactions of calcium phosphate cements in the CaHPO 4 -␣-Ca 3 (PO 4 ) 2 (DCP-␣-TCP) system have been investigated. X-ray diffraction (XRD) analyses were performed on DCP-␣-TCP cement samples of varying calcium to phosphorus (Ca/P) ratios after setting for 24 h in Ringer's solution at 37°C. XRD measurements showed that the intensity of the DCP peaks decreased linearly as the Ca/P ratio of the mixture increased. However, the intensity of the peaks of a new calcium-deficient hydroxyapatite [CDHA; Ca 9 (HPO 4 )(PO 4 ) 5 OH] precipitating phase increased linearly as the Ca/P ratio increased. Alpha-TCP was not detected after 24 h of setting in any sample. A two-phase mixture XRD model was applied to explain the results, and suitable fits were obtained between observed and expected values of the relevant peak heights. The method used for this study also can be applied to studies of the kinetic behavior of other cement systems.