Effects of additives to Pt-CeO2/activated carbon (M563) catalysts on HI decomposition were studied. Among the additives studied, it was found that the addition of Cu to Pt-CeO2 is the most effective for increasing the catalytic activity to HI decomposition. On this catalyst, almost the equilibrium hydrogen iodide (HI) conversion was achieved at temperature higher than 573K. Cu addition increased Pt dispersion by anchoring effects. Therefore, in spite of decreased Brunauer-Emmett-Teller surface area of the catalyst, dispersion of Pt was much increased by addition of Cu resulting in the increased HI decomposition activity and stability. Because formed I-2 adsorbed on the catalyst at initial ca 20hours, HI conversion was higher than that of the equilibrium one; however, after 20hours, stable HI decomposition conversion that was almost the same with equilibrium conversion was achieved in the examined temperature range.