The fuel cell is an environmentally-friendly power source due to high efficiency and cleanness. Considering safety, tractability and infrastructure, a methanol reformer is a candidate for the supply of hydrogen to fuel cell vehicles. However as CO generated by methanol reformers poisons the platinum catalysts of anodes, the operating conditions were studied in order to minimize the CO emissions from the reforming system. This study tested a methanol reforming system including a steam reformer and preferential oxidizer, established the chemical reaction rates of reforming and CO oxidation and calculated the dynamic changes in CO concentration from the reformer using a newly developed simulator. (C) 2002 Society of Automotive Engineers of Japan, Inc. and Elsevier Science B.V. All rights reserved.