Nafion/RuO2 center dot xH(2)O composite membranes were prepared by the recast method. The hydration level of RuO2 center dot xH(2)O was varied by heat-treatment of commercially available powders, and composite membranes were prepared with various RuO2 center dot xH(2)O/Nafion weight ratios. The through-plane conductivity of the membrane was evaluated in a fuel cell test station (H-2/O-2 at 80 degrees C). The through-plane conductivity decreased from 0.32 to 0.26 Omega cm(2) as a result of the introduction of RuO2 center dot xH(2)O in the composite membrane. The open-circuit voltage of the single fuel cell element is not affected by the presence of RuO(2 center dot)xH(2)O particles. Electrochemical impedance spectroscopy (four-probe method) was also used to evaluate the in-plane conductivity at 80 and 120 degrees C, and at various relative humidities (RHs) ranging from 20 to 90%. In that case, the addition of 5 wt % RuO(2 center dot)xH(2)O to Nafion causes a significant increase of the conductivity. The in-plane conductivity does not vary with RH and is unaffected by cation exchange (from H+ to Ba2+ or to Na+). This is thought to arise as a consequence of RuO(2 center dot)xH(2)O sedimentation on one side of the membrane during the casting procedure. This hypothesis was confirmed by point energy-dispersive X-ray analysis and scanning X-ray transmission microscopy that both show the presence of a thin layer (similar to 5 mu m) of RuO(2 center dot)xH(2)O on one side of the membrane. Scanning transmission X-ray microscopy also reveals that a significant fraction of RuO(2 center dot)xH(2)O is incorporated in the bulk of the membrane in the form of isolated aggregates (200-300 nm) made of smaller RuO2 center dot xH(2)O particles. These aggregates are thought to be responsible for the reduced through-plane ionic conductivity of the composite membrane observed in fuel cell test stations. (c) 2007 The Electrochemical Society.