In this study the composition of K+- and CO32--containing hydroxyapatites (KCAp’s) obtained by the hydrolysis of octacalcium phosphate (OCP), Ca-8(HPO4)(2)(PO4)(4) . 5H(2)O, was investigated as a function of the carbonate concentration c(kc) and the molar K/CO3 ratio R of the aqueous solution. OCP was hydrolyzed at 95 degrees C in K+- and CO32--containing solutions with 2.5 less than or equal to c(kc) (mM) less than or equal to 100 and 2 less than or equal to R less than or equal to 20. Physical and chemical analyses showed that the precipitates were carbonated apatites with a carbonate content ranging from 2.5 to 5.5 wt %. The samples also contained a significant amount of monohydrogen phosphate (1.3-2.2 wt %) and an appreciable amount of water (3.5-5.5 wt %) after drying at 25 degrees C under vacuum. The chemical composition of the KCAp’s can be explained by the occurrence of a main substitution mechanism [I : Ca2+ + PO43- + OH- <-> V-Ca + CO32- + V-OH] and a coupled K+ and CO32- incorporation [III : Ca2+ + PO43- <-> K+ + CO32-] where V-X stands for a vacancy in the X sublattice. The relative contribution of mechanism III with respect to mechanism I increases with increasing c(kc) and R.