The behavior of water in the gas diffusion layers of a polymer electrolyte membrane fuel cell (PEMFC) was investigated analytically. To understand the water transportation phenomena systematically, the gas diffusion layers were divided into two parts. One is the gas diffusion medium (GDM) and the other is micro-layer (ML). In this work, the GDM with different PTFE contents was intensively investigated under various single cell operation conditions. I-V performance curves of single cells were compared and analyzed with respect to water transportation in the GDM. The increased PTFE contents disturb the ejection of liquid-phase water from the electrodes to the flow channels via the GDM, especially at higher relative humidity conditions. I-V performance curves in this work can also be interpreted as an evidence that capillary force in the GDM is not the main driving force for the water transportation. Other forces, for example shear force of fluid and water evaporation etc. are more dominant driving forces, at least with in the GDM itself. This is because of the relatively larger pore diameter of the GDM compared to that of electrodes and ML. When a ML was used in the GDM, the I-V performance of fuel cell became more stabilized as well as enhanced. In regard to water management, the ML might have important roles as buffer zone which prevent serious drying and flooding of the electrode. So, when the entire gas diffusion part is designed, the capillary-force-driven water movement and the shear-force- or vaporization-driven water transportation should be considered carefully for both the ML and the GDM structures. (C) 2004 Elsevier B.V. All rights reserved.