The catalytic activity of electrochemically dealloyed PdCu3 thin films for oxygen reduction reaction (ORR) in acidic media has been studied by using a rotating disk electrode (ROE). The dealloyed PdCu3 thin films show a similar to 2.0 fold increase in the specific oxygen reduction activity over pure Pd thin films. The structure of electrochemically dealloyed PdCu3 thin films has been investigated at an atomic scale by synchrotron-based anomalous X-ray diffraction (AXRD). AXRD reveals that a Pd enriched surface layer is formed in the dealloyed film and a compressive lattice strain exists in this Pd surface layer. The enhanced catalytic activity of dealloyed Pd-Cu films for the ORR is primarily due to the compressive strain in the surface layer. We compare the structure-composition-catalytic activity relationships in dealloyed Pd-Cu thin films to related results on dealloyed Pt-Cu thin films. These studies show that dealloying and the resulting structure and the ORR activity are dependent on the nature of the noble component of alloy. (C) 2012 Elsevier B.V.. All rights reserved.