This paper studies the effect of flow channel scaling on fuel cell performance. In particular, the impact of dimensional scale on the order of 100 micrometers and below has been investigated. A model based on three-dimensional computational flow dynamics has been developed which predicts that very small channels result in significantly higher peak power densities compared to their larger counterparts. For experimental verification, microchannel flow structures fabricated with varying sizes in SU-8 photoepoxy have been tested with polymer electrolyte membrane electrode assemblies. The experimental results confirm the predicted outcome at relatively large scales. At especially small scales ~,100 mm!, the model ~which does not consider two-phase flow! disagrees with the measured data. Liquid water flooding at the small channel scale is hypothesized as a primary cause for this discrepancy. © 2004 The Electrochemical Society.