A visualization technique is employed to measure the time of breakthrough for GDL samples with different characteristics. It is shown that the time of breakthrough is reversely proportional to the hydrophobic content of the GDL sample. The effects of the GDL thickness and injection flow rate on the time of breakthrough are also studied. The results show that the capillary pressure of a thin porous medium is not independent from the time of breakthrough. Thus, it is not legitimate to calculate the saturation of the medium by multiplication of the flow rate and time of breakthrough. These findings help better understanding of flooding phenomena, which may lead to the development of more effective GDLs for PEM fuel cell. (C) 2011 Elsevier B.V. All rights reserved.