The distribution of independent water clusters within the gas diffusion layer (GDL) is an important, yet poorly understood, characteristic of polymer electrolyte membrane fuel cells. A better understanding of these water clusters would provide ex-situ invasion experiments and two-phase models with a set of validation criteria that is currently absent from the literature. Synchrotron based X-ray radiography was employed visualize liquid water emerging from the polymer electrolyte membrane fuel cell GDL. Droplet formations, entitled "breakthrough" events originated from either the channel or landing regions of the GDL. The number of breakthrough events in a given area (breakthrough density) provides insight into the size and number of independent water clusters evolving within the GDL. Water clusters were found under the flow field landings more frequently than under the gas channels. Each 1 mm(2) of projected GDL area was found to have 1-2 individual water clusters during most conditions studied, regardless of the GDL substrate or MPL type. The existence of percolating water clusters under flow field channels depended on the combination of GDL type and operating conditions employed. (C) 2015 Elsevier Ltd. All rights reserved.