The small PEMFC stack to be integrated with miniaturized fuel reformer is usually operated under very poor operating conditions such as low relative humidity and reformed gas conditions including trace amounts of CO. Hence, for the stable operation of the stack under such real conditions, the effect of reactants feeding method and stack operating conditions such as stack temperature and gas humidity were experimentally investigated and the optimal operating strategy was suggested. The external-manifolds configuration was not efficient compared to the internal-manifolds due to their poor water management. The fuel introduction to the stack by opposite directions between air and hydrogen enhanced the stack stability, particularly under low humidified conditions, by enhancing water exchange between the two fuel streams. Even 10 ppm of CO in the reformed gas deteriorated the stack performance seriously under low stack temperature or low relative humidity conditions. Hence, for the stable operation of the small PEMFC stack, particularly under practical operating conditions, air-bleeding method seems to be a promising strategy. (C) 2007 Elsevier B.V. All rights reserved.