A spectrophotometric investigation of the protonation of HVO42- has been conducted at vanadium(V) concentrations low enough (5 x 10(-5) mol dm(-3)) to prevent the formation of polynuclear ions. Equilibrium constants as well as enthalpy and entropy changes for the formation of H2VO4- and VO2+ have been determined in ionic medium : 1.0 mol dm(-3) NaCl and NaClO4, respectively. The percentage concentration of the neutral species, H3VO4, is so low (apparently <1%) that the protonation of H2VO4- to form VO2+ occurs virtually in a single step. The disproportionately great stability of VO2+ relative to H3VO4 is explained in terms of an increase in the coordination number of vanadium when the cationic species is formed. This explanation is based on a comparison of the thermodynamic quantities of the vanadium(V) species with those of various other oxyacids reported in the literature.