This study investigates the heating process of Na+- and CO32--containing apatites obtained by the hydrolysis of monetite. The results of the chemical and physical analyses of the samples reveal that all the water was removed after heating at 400 degrees C. Moreover, no other drastic constitutional changes take place. On this basis, the exact unit cell composition of these apatites could be calculated. From this composition, it was deduced that, apart from the two predominant mechanisms [Ca2+ + PO43- + OH- <-> V-Ca + CO32- + V-OH] and [Ca2+ + PO43- <-> Na+ + CO32-], a small part of the Na+-ions is incorporated in the apatite lattice independently of CO32- according to the mechanism [Ca2+ + OH- <-> Na+ + V-OH] With V-X representing a vacancy on a regular apatite lattice site occupied by X. The dependency of the contributions of these mechanisms on the Na2CO3 concentration in the hydrolysis solution differs, suggesting that the mechanisms are not completely coupled.