For the application of fuel cells in the market place, fuel cell components such as membranes, electrodes, and gas diffusion layers (GDLs) must be mass-produced to significantly reduce their cost. Variations of the physical properties of these components may affect the performance, durability, and reliability of the fuel cell operation and may result in the premature failure of the fuel cell. A segmented cell system was used to investigate the effects of variations of the GDL properties on the cell performance. A GDL containing an artificial defect was created by exchanging a standard cathode GDL at one of the inlet segments with a substitute GDL that was thinner and lacked a microporous layer (MPL). A significant local performance decrease was observed at the defective area. This performance decrease was attributed to lower GDL compression, which resulted in a reduced electrical contact at the defective segment and an increased ohmic overpotential. Spatial electrochemical impedance spectroscopy (EIS) analysis showed that the defective GDL changed water management at high current density operation. The lack of the MPL and the lower compression improved the water transport at the defective area; however, it also caused simultaneous mass-transfer limitations at the downstream segments. The combination of spatial polarization (VI) curves and EIS allowed the GDL defects to be detected and localized. (C) 2012 Elsevier Ltd. All rights reserved.