A systematic study of the solubility of the rhabdophanes LnPO4·0.667H2O (Ln = La–Dy) was performed through oversaturation and undersaturation experiments at different temperatures (298–363 K) to demonstrate the reversibility of the equilibria. The structure of the neoformed phase was controlled carefully over the entire temperature range to enable the unambiguous attribution of the solubility products and the derived thermodynamic data to the rhabdophane phases. The log K°s,0 (298 K) values for LnPO4·0.667H2O ranged from –25.6 ± 0.8 (Pr) to –24.9 ± 1.7 (Eu), and the minimum values were for Pr to Sm, whereas the ΔfG° values at 298 K varied between (–1984 ± 2) and (–2004 ± 2) kJ mol–1 whatever the lanthanide element considered, except for Eu‐rhabdophane, which presented the highest Gibbs energy of formation [(–1896 ± 2) kJ mol–1]. From the solubility data obtained at various temperatures, the enthalpies of formation of the rhabdophanes ΔfH° were found to be (–2151 ± 13) to (–2130 ± 12) kJ mol–1, excepted for that of Eu, which reached (–2057 ± 9) kJ mol–1.