The efficiency of sulfuric trioxide decomposition strongly affects to the overall efficiency of the Iodine Sulfur (IS) cycle, because the decomposition process is highly endothermic and most energy-consuming process. Therefore, we carefully selected iron (III) oxide (Fe2O3) for high temperature decomposition on the assumption of commercialization considering its proper activity, stability and price. This study starts with collecting the very basic empirical data as well as considering practical implications for actual design. Experimental results of stability test with Fe2O3 powder showed that over 65% of SO3 conversion was secured during 110 h operation. We experimentally investigated the effects related to the weight hourly space velocity (V/HSV) and gas inlet composition to SO3 conversion with pellet-type catalyst in the temperature range of 730-910 degrees C. Reaction order of SO3 decomposition was empirically verified as first-order reaction. The activation energy was also obtained as 141.4 kJ/mol under four different WHSV values. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.