The adsorption of carboxylate collectors is largely assumed to be affected by the monazite surface speciation. In order to provide key information for the understanding of this phenomenon, we propose in this study a thermodynamic model for the equilibrium of monazite-(La) crystal in an H2O-CO2 system. This model allows access to the speciation information of the dissolved lattice species. Based on this model, we identify potential chemical reactions that may affect, under certain conditions, lanthanide ions on monazite surface. Since these surface ions have vacant coordination sites, they may react with water molecules and media species to restore their coordination shells. Therefore, it has been suggested that surface lanthanide ions undergo hydroxylation and carbonation reactions under alkaline conditions. X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy were used to analyse synthetic monazite-(La) samples treated in deionised water or sodium carbonate solutions. The spectroscopy results suggest that surface lanthanide ions do tend to hydroxylate under alkaline conditions. Furthermore, under the tested conditions, no significant monazite surface carbonation was observed. These results indicate that surface lanthanide ions have limited affinity for dissolved carbonate species, contrary to what is suggested by the thermodynamic model for free lanthanide ions in the bulk.