Ca, Sr, Cd and Pb hydroxy-(Hap) and fluorapatites (Fap) and (caCd) and (CaPb) Hap have been synthesized and spectroscopically and chemically characterized, then dissolved in 9 wt% nitric acid using an isoperibol calorimeter. Dissolution of secondary products and other entities lead to standard enthalpies of formation of single-cation products. These values are, respectively, -13.305, -13.373, -8.648 and -8.261 kJ mol(-1) for Haps, and -13.545, -13.604, -8.795 and -8.529 kJ mol(-1) for Faps. Replacement of OH by F decreases the lattice volume and increases the stability of the structure. In the Hap and Fap series, the smaller the lattice volume, the worse the stability, except for Pb products. Dissolution of solid solution Haps leads to enthalpies of mixing of the limit products. The results are in agreement with the preferable occupancy of site II by Cd or Pb up to 40% and 45%, respectively.