The kinetics of the oxygen reduction reaction on epitaxial thin films of Pd on Pt(111) was studied using the rotating ring-disk electrode technique. The results for the pseudomorphic monolayer of Pd are compared to those of the unmodified Pt(111) surface, both surfaces having nominally identical 2D structures. In an electrolyte containing a nonabsorbing anion, 0.5 M HClO₄, the pseudomorphic layer of Pd has a somewhat lower (ca. factor of 2) activity than Pt(111). In an electrolyte containing an anion which is strongly adsorbing on Pt(111), 0.5 M H₂SO₄, the inhibition due to anion absorption is even greater on the pseudomorphic monolayer of Pd than on the unmodified Pt(111) surface. This additional inhibition is attributed to the difference in the potential of zero charge (pzc) between the two surfaces, the Pt(111) having the more positive pzc.