Processes taking place during the reaction of solutions of barium and strontium phosphates with sodium orthophosphate were studied in melts based on LiCl–KCl eutectic (550°C), NaCl–KCl–CsCl eutectic (650°C), NaCl–KCl equimolar mixture (750°C), as well as in NaCl–KCl–CsCl–LaCl3 melt (650°C) in an inert atmosphere. Chemical and X-ray diffraction analyses were used to determine composition of the sparingly soluble phosphates formed. The effect of the molar ratio of phosphate-ions to Sr2+/Ba2+ on the phase composition of the precipitated phosphates was studied. The conditions required for quantitative precipitation of strontium and barium from the melt in the form of phosphates were determined.