Two titanium oxide catalyst supports for the oxygen electrode of unitized regenerative fuel cells were compared by electrochemical impedance spectroscopy (EIS) techniques. One was a commercial Ebonex sample, which is a mixture of Ti4O7 and other conductive Magneli phases, and the other was a niobium-doped rutile oxide, Ti0.9Nb0.1O2. Both supports were loaded with a mixed Ir-Ru-Pt oxide catalyst and conditioned anodically. The impedance spectra shows a resistive component which can be assigned, using transmission line modeling of the catalyst/support/electrode structure, to the series resistance of the support particles. Although both supports increase in resistance with anodic conditioning, the increase is greater for Ebonex. This increase in support resistance is consistent with oxidation of Ti-III to Ti-IV, which appears to be reversible in the case of the Nb-doped rutile material but irreversible with Ebonex. (C) 2003 The Electrochemical Society.