The mechanism of oxygen reduction on a bare palladium surface was studied in 0.1 M LiOD solution using a rotating Pt ring-Pd disk electrode and programmed cyclic voltammetry. A polarization curve, obtained from linear sweep voltammetry (- 0.1 --> - 0.7 V(SCE)) starting after the Pd disk had been prereduced at - 0.5 V(SCE) for 300 s and then held at - 0.1 V(SCE) for 120 s, showed a Tafel slope of - 52 mV dec(-1). A polarization curve, obtained from the reverse sweep of cyclic voltammetry (- 0.5 --> - 0.1 --> - 0.7 V(SCE)) started immediately after the electrochemical prereduction treatment, showed characteristics of reversible reaction under diffusional control. These results indicated that the spontaneous adsorption of oxygen species may possibly be avoided by using programmed cyclic voltammetry. From the analysis of Levich plots, an electron transfer number for the oxygen reduction reaction on bare palladium was determined to be about 2.9. The dependences of disk and ring currents on electrode rotation speeds were examined. On the basis of the experimental results, it is suggested that about one-third of oxygen molecules reacting at the bare palladium surface are reduced directly to OD- via the four-electron pathway and the rest of the molecules are reduced to deuterium peroxide via the two-electron pathway.