Pt1-xTax (0 <= x <= 1) thin-film libraries were prepared using combinatorial dc magnetron sputtering on a variety of substrates. For x < 0.5, the materials adopted a face-centered cubic structure and for x > 0.5 a nanocrystalline phase of Pt1-xTax was observed. The bulk composition of the films obtained by electron microprobe before and after treatment in 0.5 M H2SO4 at 80 degrees C showed no detectable dissolution of Ta at any x in Pt1-xTax. X-ray photoelectron spectroscopy of acid-treated samples revealed that for regions with high Ta content (x > 0.5), full oxidation of surface Ta had occurred. The samples, when used as oxygen-reduction electrocatalysts, also showed corrosion stability; however, the addition of Ta significantly lowered the activity towards oxygen reduction reaction for x > 0.1 in Pt1-xTax. (c) 2006 The Electrochemical Society.