Oxygen reduction at RuO2-coated titanium electrodes prepared by thermal decomposition was investigated by employing cyclic voltammetry and rotating-disc electrode techniques. Cyclic voltammetric results indicated that oxygen reduction was catalysed by at least some hydrous oxyruthenium species, (i.e., Ru(III)) at low polarization range (E > -0.45 V) and by at least some low oxide states of ruthenium species at high polarization (E < -0.45 V). On the basis of measurements using a rotating disc electrode together with polarization curves, Tafel slopes and stoichiometric number determinations, two mechanisms for oxygen reduction on an RuO2-coated titanium electrode are proposed.