In this work, a transparent assembly was self-designed and manufactured to perform ex situ experimental study on the liquid water removal characteristics in PEM fuel cell parallel flow channels. It was found that the dominant frequency of the pressure drop across the flow channels may be utilized as an effective diagnostic tool for water removal. Peaks higher than I Hz in dominant frequency profile indicated water droplet removals at the outlet, whereas relatively lower peaks (between 0.3 and 0.8 Hz) corresponded to water stream removals. The pressure drop signal, although correlated with the water removal at the outlet, was readily influenced by the two phase flow transport in channel, particularly at high air flow rates. The real-time Visualization images were presented to show a typical water droplet removal process. The findings suggest that dominant frequency of pressure drop signal may substitute pressure drop as a more effective and reliable diagnostic tool for water removal in PEM fuel cell flow channels. (C) 2009 Elsevier B.V. All rights reserved.