For portable applications, the, characteristics of passive air-breathing PEMFCs were investigated by examining effects of cathode open area and relative humidity on the cell performance. Among the single cells with cathode open area from 52 to 94%, the single cell with the open area of 77% exhibited the highest performance. The cell performance was improved with increasing RH of atmosphere from 20 to 100% in the low current region while lowered in high current region. Those results were related with the mass transport of oxygen from the atmosphere to the catalyst layer and the degree of membrane hydration determining the ionic conductivity of the membrane. (c) 2005 Published by Elsevier B.V.