The solubility of the potassium metavanadate and ammonium metavanadate in the KVO3 + NH4VO3 + H2O system has been measured over a temperature range 293 K to 323 K. On the basis of the results, a section of the polytherm has been drawn. Additionally, the densities of the solutions have been measured. These results have led to the construction of the equilibrium diagram for the KVO3 + NH4-HCO3 + KHCO3 + NH4VO3 + H2O system in the oblique projection according to Janecki, The optimum conditions for conducting the carbonization process of the KVO3 ammonium solutions in order to obtain KHCO3 are described.