Herein, we present a comprehensive study on the dissolution behaviour of two sodium–cerium(IV) phosphate phases synthesised hydrothermally from CeO2 nanoparticles: crystalline Na2Ce(PO4)2 and nanocrystalline NaCe2(PO4)3. For the first time, experimental dissolution data were obtained for both compounds over a wide pH range (1.5–10) under long-term equilibration. The crystalline phase undergoes pH-dependent transformation, including recrystallisation at a near-neutral pH and the formation of secondary CeO2 nanoparticles above pH 7. In contrast, the nanophase NaCe2(PO4)3 exhibits exceptional structural and chemical stability, showing no signs of recrystallisation, phase transformation, or CeO2 formation, even after extended ageing. The experimental results help refine the thermodynamic stability conditions for cerium phosphate and oxide phases, providing insights into the reversible transformation pathways between CeO2 and Ce(IV) phosphates as governed by pH.
