A transparent PEMFC with a single straight channel was designed to study liquid water transport in the cathode channel. The pressure-drop between the inlet and outlet of the channel was measured and used as a diagnostic signal to monitor liquid water accumulation and removal. This method was non-destructive for the fuel cell, and is capable of monitoring the water droplet buildup and removal in the channel on-line directly, and giving real-time liquid water buildup information. The proper velocity for liquid water removal can be determined according to the pressure-drop curve, which was very helpful to design a flow field and to optimize fuel cell operation. Under the study conditions, and to ensure liquid water discharge, the gas velocity should not lower than 2, 3 and 5 m s(-1) for 600, 1000 and 1200 mA cm(-2), respectively. The results were further verified by visualization in a transparent PEMFC. (c) 2006 Elsevier B.V. All rights reserved.