Cationic contamination of polymer electrolyte fuel cells is known to cause performance degradation, particularly due to displacement of protons by foreign cations in the ionomer. Recent findings however, show that mass transport is a critical mechanism in the contamination process. X-ray computed tomography is used to examine the salt precipitation within the cathode diffusion media following in situ cation contamination experiments. Statistical analysis of the tomographic data is performed to examine the salt distribution inside a contaminated dual layer carbon paper substrate (GDL)-MPL. Results confirm the presence of salts on the surface, which is visually observable, but also show subsurface salts that are present up to the GDL-MPL interface. This new finding shows that cationic salts can precipitate in the macroporous carbon paper substrate, and the hydrophobic microporous MPL acts as a barrier to restrict cationic transport from reaching the fuel cell catalyst layer and membrane. (C) 2017 The Electrochemical Society. All rights reserved.