A segmented cell system was used to study the impact of localized variations of GDL polytetrafluoroethylene (PTFE) on the spatial and overall fuel cell performance. An artificial defect was created by exchanging the standard cathode GDL treated by 13 wt% PTFE and located at segment 4 with a defective GDL containing 8, 17, 26, or 35 wt% PTFE. Operation under wet (100/50% RH) and dry (32/32% RH) conditions demonstrated the same trend, in which an increase in the PTFE loading caused a local performance decrease at high current density; however, a low humidity mitigated the impact of PTFE loading defects. Spatial impedance spectroscopy data together with analysis of the polarization curves revealed that such a performance decrease is attributed to an increase in the mass transfer limitations ascribed to the changes in textural (decrease in porosity, pore volume, and pores size), structural, and even chemical properties of the GDLs with PTFE loading. The impacts of PTFE loading on the surface structure and the morphology of the GDLs were studied by digital image processing of the SEM images, which were deconvoluted into low- and high-frequency components. Higher hydrophilicity zones were also identified and ascribed to partially fluorinated species detected by XPS. (C) 2013 The Electrochemical Society. All rights reserved.