The behavior of water in the air-water flow inside a serpentine channel for a proton exchange membrane (PEM) fuel cell was investigated using the FLUENT software package. The volume-of-fluid (VOF) model was adopted to track the dynamic air-water interface. Five cases with varying initial water phase distribution corresponding to different fuel cell operating conditions were numerically simulated to obtain a better understanding of water behavior inside a serpentine micro-channel. Results show that the bend area of a serpentine flow channel has significant effects on the flow field, which in turn affects the air-water flow and water liquid distribution inside the channel or along the interior channel surfaces. The simulation results also indicate that water flooding could occur in the "after-bend" section of a micro-channel. For the case with larger amount of water in the two-phase flow, the simulation shows that the "after-bend" water distribution might block the reactant supply to reaction sites and, in some extreme situations, might block the reactant transport inside the flow channel, thus decreasing fuel cell performance. (c) 2005 Elsevier B.V. All rights reserved.