Nafion/RuO2.xH2O composite membranes were prepared by the recast method. The hydration level of RuO2.xH2O was varied by heat-treatment of commercially available powders, and composite membranes were prepared with various RuO2.xH2O/Nafion weight ratios. The through-plane conductivity of the membrane was evaluated in a fuel cell test station (H2O at 80ºC ). The through-plane conductivity decreased from as a result of the introduction of RuO2.xH2O in the composite membrane. The open-circuit voltage of the single fuel cell element is not affected by the presence of RuO2.xH2O particles. Electrochemical impedance spectroscopy (four-probe method) was also used to evaluate the in-plane conductivity at 80 and 120ºC , and at various relative humidities (RHs) ranging from 20 to 90%. In that case, the addition of RuO2.xH2O 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 RuO2.xH2O 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 (~ 5µm) of RuO2.xH2O on one side of the membrane. Scanning transmission X-ray microscopy also reveals that a significant fraction of RuO2.xH2O is incorporated in the bulk of the membrane in the form of isolated aggregates (200-300 nm) made of smaller RuO2.xH2O 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.