One of the key reactions for efficient hydrogen production using the water-splitting iodine-sulfur (IS or S-I) process is the Bunsen reaction (SO2 + I-2 + 2H(2)O = H2SO4 + 2HI). The Bunsen reaction was examined using an electrochemical cell with a cation exchange membrane as the separator. The optimal molalities of anolyte and catholyte were evaluated by total thermal efficiency using the heat/mass balance of the IS process. The I-2/HI ratio had little effect on the required total voltage; the I-2/HI ratio can be reduced to 0.5 without decreasing the total thermal efficiency. On the other hand, HI and H2SO4 molality greatly affected the total thermal efficiency. The total thermal efficiency increased with increasing HI molality up to 16.7 mol kg(-H2O)(-1) and the maximum thermal efficiency was found at 15.3 mol kg(-H2O)(-1) of H2SO4. Membrane resistances are very important parameters affecting the efficiency. The total thermal efficiency increased by 3.0% at a current density of the electrochemical cell of 10.0A dm(-2) by increasing the operating temperature from 313 to 363 K. (c) 2005 Elsevier Ltd. All rights reserved.