This work proves that the reverse-current phenomenon occurs in polymer electrolyte fuel cells even in small regions of channels and lands on a length scale of fractions of a millimeter. The effect of the flow field design, proposed mitigation strategies, and operation conditions are evaluated by using submillimeter resolved current distribution measurements during start-up and shutdown experiments. The results demonstrate that a high gas flow rate and a short fuel gas introduction time cannot avoid mass transport limitations underneath the ribs, which cause temporarily an inhomogeneous gas distribution over the fuel electrode and in-plane current transients in the lateral direction. (c) 2010 The Electrochemical Society. [DOI: 10.1149/1.3518520] All rights reserved.