NOVELTY - Sulfur dioxide is liquefied at pressure of 5.5 MPa or more in the presence of water to provide a reaction phase which is supplied to anode (46) of electrolyzer (16). The liquid sulfur dioxide (40) is immiscible in water. A liquid trioxide is formed by oxidizing liquid sulfur dioxide to liquid trioxide in the anode. A liquid phase containing dense sulfuric acid solution (52) and liquid sulfur trioxide is formed in electrolyzer from sulfur trioxide and excess water. The liquid is recovered from electrolyzer, and hydrogen gas (6) is recovered from a cathode (44).    USE - For operation of electrolyzer used for producing hydrogen gas and performing Westinghouse Sulfur Process (claimed).    ADVANTAGE - The electrolyzer increases process energy efficiency of sulfur dioxide processes, and reduces amount of water required economically and efficiently.    DETAILED DESCRIPTION - Sulfur dioxide is liquefied at pressure of 5.5 MPa or more in the presence of water to provide a reaction phase which is supplied to the anode of an electrolyzer. The liquid sulfur dioxide is immiscible in water whose amount is less than or equal to2 moles per mole of liquid sulfur dioxide. A liquid trioxide reactive to excess water is formed by oxidizing liquid sulfur dioxide to liquid trioxide in the anode and 2H+ and 2e- are provided at or near the cathode of electrolyzer. A liquid phase containing dense sulfuric acid solution and liquid sulfur trioxide both denser than water and liquid sulfur dioxide, is formed in the electrolyzer from sulfur trioxide and excess water. The liquid is recovered from the electrolyzer, and hydrogen gas at pressure of 5.5 MPa or more is recovered from the cathode of the electrolyzer by reaction of 2H+ and 2e-.    An INDEPENDENT CLAIM is also included for method of performing Westinghouse Sulfur Process.    DESCRIPTION OF DRAWING(S) - The figure shows the block diagram of the electrolyzer operating at high pressure to provide liquid sulfur trioxide.    Hydrogen gas (6)    Electrolyzer (16)    Sulfur dioxide steam (40)    Cathode (44)    Anode (46)    Dense sulfuric acid solution (52)