The efficacy of using ZnO as an absorber for the removal of H2S from small fuel processor steam reformate streams was examined. At temperatures below 300 degrees C, H2S can in principle be reduced to below 100 ppbv, required for safe operation of PEM fuel cells. The ZnO adsorbent performed predictably based on ZnO-H2S-ZnS-H2O equilibria with steam, hydrogen, and CO2 in the feed. However, addition of CO even at levels as low as 1 vol% drastically lowered the sulfur removal capability of the ZnO. This is consistent with the formation of COS by the reaction H2S + CO = H-2 + COS- ZnO is not an efficient absorber for COS. Indirect evidence is also provided for the formation of COS via the reaction CO2 + H2S = COS + H2O, which can occur in special cases when CO2 is present but neither H2O nor CO is present in the feed. The potential for COS formation compromises the ability of ZnO to deliver very low sulfur concentration reformate. (C) 2006 Elsevier B.V. All rights reserved.