The accelerating role of water in the insertion of hydrogen into tungsten trioxide WO3 was studied in non-aqueous aprotic and/or protic reaction media and in aqueous solutions of the sulfuric acid. The activation energies of hydrogen insertion in various reaction media were estimated. The partial molar heat of hydrogen insertion DELTAH = 8.7+/-1.2 kJ/mole of H0.45WO3 was evaluated.