Coal-based activated carbon (AC-COAL) catalysts subjected to acid treatment were tested to evaluate their performance on hydrogen-iodide (HI) decomposition for hydrogen production in sulfur-iodine (SI or IS) cycle. The effects of acid treatment on catalysts and the relations between sample properties and catalytic activities were discussed. The AC-COAL obtained by non-oxidative acid treatments had the best catalytic activity. However, the catalytic activity of AC-COAL decreased after the treatment of nitric acid. Higher surface area, higher carbon contents, lower ash contents and fewer surface oxidation groups contributed to the catalytic activity of ACs. HI decomposition on the AC surface itself may be due to high densities of unpaired electrons associated with structural defects and edge plane sites with similar structural ordering. Moreover, the oxygen-containing groups reduced the electron transfer capability associated with the basal plane sites. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.