The electrochemical behavior of Pt1-xMnx (0.1 < x < 0.8) catalysts prepared by magnetron sputtering were studied extensively using the rotating disk electrode technique, electron microprobe, X-ray photoelectron spectroscopy and X-ray diffraction. The data suggests that Mn forms an oxide layer at the surface of the catalyst upon contact with air. Mn is preferentially drawn out of the intermixed catalyst into the oxide layer, depleting the core of Mn content. Without significant amounts of Mn present at the core of the catalyst, de-alloying reactions have less effect on the electrochemically active surface area compared to the case of Pt1-xNix, for example. High and low Mn content alloys have about the same Pt-area specific current densities, j(specific), so it is likely that all samples have the same catalytic surface composition, namely Pt. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.019206jes] All rights reserved.