In situ X-ray absorption spectroscopy (XAS) measurements were carried out on commercial carbon-supported Pt, Ru and Pt-Ru alloy electrocatalysts as well as a Pt/Ru mixture electrocatalyst in a specifically designed reactor/furnace set-up at the Pt L-3 and the Ru K edge. The catalysts were heated to 100 degrees C and subjected to different atmospheres -5% H-2/N-2, 5% CO/N-2, N-2, air-of interest for fuel cell operation. X-ray absorption spectroscopy was used to follow changes in the catalyst structure, most importantly particle growth, oxidation, and (de-)alloying. Alloying is observed to be advantageous, as it decreases particle growth and oxidation tendency in the catalysts. Initially, all electrocatalysts contained large amounts of the respective oxides, as indicated by pronounced white-line intensities in the XANES spectra, whereas the catalysts were reduced to the metallic state upon exposure to hydrogen. In CO atmosphere, however, ruthenium oxides remain stable, depending on the Pt to Ru site distribution: it is assumed that Pt in contact with Ru acts as a "catalyst" for the reduction of ruthenium oxides and strengthens the Ru-CO bond favouring it over Ru-O (ligand effect). Consequently, the share of ruthenium oxides in the Pt-Ru alloy decreases in 5% CO/N-2, whereas for the Pt/Ru mixture and the pure Ru it does not change significantly. (c) 2006 Elsevier B.V. All rights reserved.