Calcium hydroxyapatite (Ca-HA) is a versatile material which can be used in several applications. This study aimed to investigate the one-step synthesis of Ca-HA using solely calcium carbonate as-is and orthophosphoric acid as widely available and low cost initial reactants. The stirring rate and quantity of water had only some slight effects on the reaction rate. On the other hand, the reaction temperature influenced strongly the decomposition of the initial calcium carbonate particles and therefore the composition of the solids obtained. At 25-60 degrees C, high contents of calcium carbonate remained in the solid products even after a long reaction time of 168 h. At 80 degrees C, the decomposition of calcium carbonate was nearly complete after 24 h of reaction. Brushite (CaHPO4 center dot 2H(2)O) and octacalcium phosphate (OCP, Ca-8(HPO4)(2)(PO4)(4)center dot 5H(2)O) were the main intermediates of the reaction. When the decomposition of calcium carbonate was complete (80 degrees C, 24 h), stoichiometric Ca-HA was obtained, with some little amounts of carbonate-containing apatites of both A-type and B-type. Carbon dioxide was the only by-product of the reaction, and no water rinsing was required for the purification of products.