Uniform flow distribution is critical to obtaining high performance in many heat and mass transfer devices. it also plays an important role in the effective operation of a proton exchange membrane fuel cell (PEMFC). Presently there are a few theoretically based models available for predicting flow distribution in individual fuel cell channels and across fuel cell stacks in PEMFCs, but little or no experimental data has been published on the actual flow rates measured in individual channels. This is mainly because of the lack of experimental techniques available to measure the instantaneous flow rates in parallel channels. In this work, a novel technique based on the entrance region pressure drop measurements is presented for monitoring fluid flow maldistribution in individual channels. The method is validated using liquid water flow in a test section with four tubes in parallel, and then applied to assess the air flow maldistribution in PEMFCs using (a) an ex-situ experimental setup simulating the two-phase flow in parallel channels, and (b) an in-situ experimental setup with an operating fuel cell. While an almost uniform air distribution is obtained for the parallel channels with an impermeable backing (plastic sheet), severe maldistribution is observed for the same channels with porous GDL backing. The maldistribution caused by the water blockage in an ex-situ test setup is further investigated and the results are verified by the high-speed images of the two-phase flow in channels. The technique has also been applied in an in-situ experimental setup to obtain the flow maldistribution under electrochemical reaction conditions in the presence of two-phase flow in the cathode side gas channels. (C) 2008 Elsevier Ltd. All rights reserved.