Abstract <p>Britholites are phospho-silicate-apatites comprising rare earth elements; they are seen as potential candidates for the immobilization of actinides and fission products. In this work, the thermochemical stability of an apatite solid solution was studied, named neodymium–enclosing silicate–oxyapatites or britholites Ca_{10–<i>x</i>}Nd_<i>x</i>(PO₄)_6−<i>x</i>(SiO₄)_<i>x</i>O having different substitution degree <i>x</i>, with 1 &lt; <i>x</i> &lt; 6. The calorimetric investigation involved the application of a swinging differential Tian–Calvet calorimeter to measure the enthalpies of a solution in a nitric acidic aqueous solution (46 wt %) at a temperature of 298.15 K. The determination of the formation enthalpies of britholites was achieved by a thermochemical cycle and further experimental investigations. Such experimental results are readily usable thermodynamic data for industrial and geochemical studies on the stability of apatites.</p>