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The composition of Hydroxyapatite (HAp) is expressed by the formula Ca10(PO4)6(OH)2. Many reports have been published on the synthesis of HAp in which Ca ions are substituted by various cations (e.g., Sr, Ba). On the other hand, studies on the synthesis of sulfate-ion-substituted hydroxyapatite (SAp) have rarely been conducted. The present study investigated the conditions for the synthesis of SAp from amorphous calcium phosphate (ACP, Ca3(PO4)2• nH2O) as the starting material, which can readily incorporate various ions into its structure. Sodium sulfate (Na2SO4) was added to ACP, and then, the mixture was hydrothermally processed at 220°C for 3 h. SAp obtained under these conditions had a Ca-deficient-type HAp structure. The SO4/PO4 molar ratio in SAp increased with increasing amounts of added Na2SO4, reaching a maximum value of 0.5, meaning that 1/3 of the PO4 3– ions contained in HAp were substituted by SO4 2– ions.