This study investigates the stoichiometry and the thermal stability of K+ and CO32--containing apatites (KCAp’s) obtained by the hydrolysis of monetite. The analysis results of the samples after drying reveal that the KCAp’s start to lose carbonate at temperatures less than or equal to 400 degrees C. The predominant substitution mechanisms for the K+- and CO32- incorporation in calcium hydroxyapatite are [Ca2+ + PO43- + OH- <-> V-Ca + CO32- + V-OH] and [Ca2+ + PO43- <-> K+ + CO32-], where V-X stands for a vacancy in the X-sublattice. Moreover, a small part of the CO32- ions are presumably incorporated according to [Ca2+ + 2PO(4)(3-) <-> V-Ca + 2CO(3)(2-)]. A comparison of the contributions of these fundamental mechanisms with the results for precipitated Na+- and CO32--containing apatites shows that no intrinsic coupling whatsoever exists between these mechanisms.