Solubility data of the KVO3 + NH4HCO3 + NH4VO3 + KHCO3 + H2O system at 303 K were determined under varying pressure conditions. The results were used to construct a phase diagram in the oblique projection according to Janecke's method. At constant p and T this diagram includes two invariant points, five double saturated liquid curves, and four crystallization fields corresponding to KVO3, NH4HCO3, NH4VO3, and KHCO3. It has been found that ammonium meta-vanadate is a sparingly soluble salt. NH4VO3 and KHCO3 compose the stable pair of salts, whereas KVO3 and NH4HCO3 form the unstable salt-pair. A thorough knowledge of the solubility phase diagram for this reciprocal quaternary salt system is the theoretical basis of the carbonation process of the potassium meta-vanadate saturated ammonia solution. © 2004 Elsevier B.V. All rights reserved.