Thermochemical hydrogen production by the iodine-sulfur process decomposes water into hydrogen and oxygen by combining the chemical reactions of iodine (I) and sulfur (S). Two types of acids are produced through the Bunsen reaction (I-2+SO2+H2O=2HI+H2SO4). To improve the performance of this reaction, ion-exchange membranes for the membrane Bunsen reaction should be developed. In the present study, a cation-exchange membrane was prepared using a radiation-graft polymerization method. It was found that a divinylbenzene crosslinking procedure was very effective in reducing water permeation through the membrane, and the membrane Bunsen reaction was successfully carried out by using the developed crosslinked membrane. Moreover, sulfur deposition during the membrane Bunsen reaction was low for the crosslinked membrane. Therefore, the developed crosslinked membrane is a potential candidate as a cation-exchange membrane for the membrane Bunsen reaction.