The activation of platinum‐ruthenium (Pt‐Ru) fuel cell catalysts, which were made by supporting nanoscopic colloidal precursors on conductive carbon (Vulcan XC72) is brought about by reactive annealing (“conditioning process”) at 250–300 °C, in a particular set‐up. The tetraalkylammonium (A) or triorganoaluminium (B) protecting shell is cleaved from the Pt‐Ru metal surface in three mandatory steps under flowing argon, oxygen, and hydrogen (30 min each). Ex‐situ TEM micrographs were used to monitor changes in particle size during this conditioning process. Additionally, in‐situ X‐ray absorption spectroscopic measurements on the Pt L_III‐edge provided insight into the processes occurring in the metallic core of the particles. While a small increase in size is found with conditioning, the dominant structural change in both catalyst types (A and B) is a reduction in the disorder inside the metallic particles, particularly in those having organoaluminium‐protected nanoparticles (B). During oxidation under flowing oxygen, platinum atoms only undergo oxidation in the bimetallic Pt‐Ru catalyst type (B) while the platinum atoms in the bimetallic Pt‐Ru catalyst (A) remain strictly zerovalent. This difference may be attributed to differences in the internal structure of the as prepared colloidal particles (A and B) as evidenced from Pt L_III EXAFS‐data and XANES spectra.