The 4-D imaging and quantification of the desaturation of an initially flooded gas diffusion layer (GDL) with a serpentine gas flow channel by synchrotron radiography is presented. An imaging area with a diameter of 10 mm allows for the study of how the natural anisotropy of a GDL affects the desaturation profile. The GDL is progressively spatially segmented from the overall domain down to sections of the individual channels and ribs. Temporal saturation profiles and desaturation rates are presented for each step of this progressive segmentation. Although the desaturation of the overall domain initially appears to be a constant and steady process with a desaturation rate of 0.0030 mu L cm(-2) s(-1), segmented results display heterogeneous behavior with over two-fold differences between areas of the flow field. Segmented saturation surface plots are presented that spatially show how this heterogeneity occurs within the entire domain. Results indicate that initial conditions, anisotropic material properties, and flow field geometry all play a key role in determining local desaturation behavior. The larger domain considered here provides valuable data for future modeling studies that combine continuum studies of flow in the gas flow channels with GDL pore network models that consider two-phase flow behavior.