Alkali-rich F-Cl-CO₂-bearing aqueous fluids are thought to be common ore-forming fluids of carbonatite-associated rare earth element (REE) deposits. Solubilities of synthetic monazite-(La) and xenotime-(Y) end-members in such fluids were determined at 800 °C and 0.5 GPa in a series of piston-cylinder experiments. At run conditions, solutions of NaCl/KCl (1.9 mol/kg) ± CO₂ (5% mole), NaCl/KCl (1.7 mol/kg) ± NaF/KF (0.2 mol/kg) were trapped as synthetic fluid inclusions in quartz, and subsequently analyzed by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). The resultant concentrations in these fluids are extremely low, ranging from 2.92 × 10^− 6 to 3.05 × 10^− 5 mol La/kg H₂O and 3.97 × 10^− 6 to 2.74 × 10^− 5 mol Y/kg H₂O. Such low solubilities indicate that fluids expelled from carbonatite melts contain insufficient phosphate and REEsimultaneously to form REE deposits. Mixing REE-bearing fluids with phosphate-bearing aqueous fluids or interacting with apatite in the wall rock may be a key control on the deposition of REE phosphate minerals. In addition, our results show that, at nearly equal total ligand molality, solubilities of La and Y are both higher in F or CO₂-bearing solutions than in Cl-only solutions, and the highest solubilities obtained overall were in NaCl + NaF solutions, which significantly increase by a factor of 9.4 for La and 6.1 for Y relative to NaCl-only solutions. Moreover, the solubility differences between xenotime and monazite in F/CO₂-bearing solutions, shown as Y/La (molality ratio), are higher in K-bearing solutions than in Na-bearing solutions. Y/La in the NaCl + F/CO₂ solutions are close to 1, whereas in the KCl + F/CO₂ solutions Y/La increase to 2.9 and 2.0, respectively. These results indicate that during the evolution of carbonatitic fluids, enrichment of Na and F can significantly increase solubilities of all REE, whilst enrichment of K may promote the fractionation of HREE from LREE in the presence of F or CO₂.