The molal enthalpies of solution of VOSO₄•2.76H₂O(s) with various molalities in water and in aqueous sulfuric acid (0.5 mol•kg^－1), Δ_sH_m, were measured by a solution-reaction isoperibol calorimeter at 298.15 K. The value of －Δ_sH_m in aqueous sulfuric acid was lower than that in water, which shows that energy state of VOSO₄ in aqueous H₂SO₄ was higher than that in pure water. An improved Archer s method was put forward in terms of ionic association equilibrium of VOSO₄ and second dissociation equilibrium of H₂SO₄. Using the improved method, the standard molal enthalpies of solution ＝－29.18 kJ•mol^－1 in pure water and ＝－25.79 kJ•mol^－1 in aqueous sulfuric acid were obtained, respectively. According to calculation, the molality of VO^2＋ was larger in VOSO₄-H₂SO₄ mixed solution than that in water.

Key words: vanadium redox, battery, enthalpy of solution, vanadyl sulfate, sulfuric acid