The Bunsen reaction (SO2 + I-2 + 2H(2)O = H2SO4 + 2HI) was examined by an electrochemical cell featuring a cation-exchange membrane as the separator, using sulfuric acid dissolving sulfur dioxide as the anolyte and hydriodic acid dissolving iodine as the catholyte. In galvanostatic electrolysis, the molality of H2SO4 in the anolyte and that of HI in the catholyte were increased up to 17.8 and 14.9 mol kg(H2O)(-1), respectively. These concentrations were far higher than those that were obtained by the Bunsen reaction carried out in the presence of a large amount of iodine (such as I-2/HI = 4). 12 concentration after the concentration procedure was at I-2/HI = 0.95, which is lower than the reported value. 12 is one of the recycling agents in the IS process. HI and H2SO4 were successfully concentrated under low 12 concentration. The concentration of HI agreed with the calculated value, based on the amount of electricity consumed, indicating high current efficiency. Heat/mass balance using this type of electrochemical cell through the IS process is discussed for evaluation of the reactor. The thermal efficiency to produce hydrogen was calculated at 42.1%, without heat recovery for electricity, by optimizing HI and H2SO4 concentrations after application of the electrochemical cell. (C) 2004 American Institute of Chemical Engineers.