A study is made of the transport phenomena related to fluid flow through proton exchange membrane fuel cells (PEMFCs) that have microchannels. Specifically, transport is investigated in channels of 100 mu m and smaller, which are constructed using a structural photopolymer (SU-8). It has been suggested by other workers that fuel cell micro-channels may suffer from flooding if flow channels are too small (< 100 mu m). In order to confirm or reject this hypothesis, the electrochemical impedance technique has been employed to measure the mass transfer resistance. In smaller channels, mass transportation resistance is found to increase due to flooding, but that this effect may be offset by a reduction in dead zone area. (c) 2006 Elsevier B.V. All rights reserved.