Quantification of liquid water saturation distribution in a diffusion medium is critical to establishing a basic understanding of the two-phase flow and flooding occurrence in proton exchange membrane (PEM) fuel cells. We have used X-ray microtomography to obtain high-resolution (10x10x13.4 mu m), three-dimensional images of liquid water distribution in a gas diffusion layer (GDL) during gas purge. We report on temporally resolved liquid saturation profiles across the GDL thickness and demonstrate the feasibility of using X-ray microtomography to quantify liquid water distribution at the component level. The results show that the drying rate decreases exponentially with purge time and no significant liquid water removal takes place after 6 min of purge, at room temperature. (c) 2006 The Electrochemical Society.