The drainage process in the gas diffusion layer (GDL) of proton exchange membrane (PEM) fuel cells is investigated with an ex situ experimental approach. The drainage process is investigated for stable displacement, capillary fingering, and viscous fingering flow regimes. Liquid water and air percolation within the plane of the GDL is visualized with a CCD camera and at different GDL compressions. Experiments were carried out for carbon paper and carbon cloth samples to visualize water and air percolations. The percolation images are shown at four stages of percolation, defined by a nondimensional time scale. In addition, images are analyzed to obtain the wetted area as well as the wetted perimeter of percolations. The in-plane percolation characteristic length, Gamma, is defined as the ratio of the wetted area to the wetted perimeter. Image processing reveals that Gamma decreases from stable displacement to capillary fingering flow regime, confirming the formation of multiple fingers in the capillary fingering flow regime which increase the wetted perimeter. This in-plane percolation characteristic length was even smaller for viscous fingering flow regime, an indication of multiple conduits and fingers formed during the percolation of air. The data gathered in this study can be used to validate pore-network models for fluid percolation within a porous media.