Water flooding and flow dynamic characteristics in the flow field of a transparent proton exchange membrane fuel cell (PEMFC) consisting of an optical window were studied experimentally. in situ observations of the liquid water build-up and two-phase flow patterns inside the flow field were made. The effects of cell temperature, cathode flow rate and operating time on the flow patterns and cell performance were studied, respectively. Experimental results indicate that the condensation of liquid water in the flow field decreases with the increase of cell temperature. Increasing cell temperature to enhance cell performance depends on two aspects: enhancing the electrochemical kinetics and activity of the catalyst and reducing condensation of liquid water to improve mass transport. Increasing oxygen flow rate can remove more liquid water out of the cathode flow field and enhance cell performance. The distinguished flow patterns in low-Bo, low-Su environment were observed. The different flow patterns of slug flow with a clear phase boundary, core-annular flow and a transition flow pattern were found to depend on the oxygen flow rates (cathode Reynolds number). Experimental results also show that the water generation continues under operating conditions during the operation time. The water droplets increased in size with operation time, and some water droplets were able to grow to a size of 0.7-0.8mm in diameter, comparable to the cross-sectional dimension of the flow channels. Finally, the water droplets coalescence formed water columns in the flow channels. Mass transfer limitation due to liquid water column clogging leads to poor performance of PEMFCs. (C) 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.