In situ XRD has been used to study both unsupported and silica-supported clusters PtRu[N(Oct)(4)Cl](5) (Pt:Ru approximate to 1:1). Structural information has been gained by Debye function analysis. For the unsupported clusters, the best fit simulation gives a 50/50 mixture of colloid particles with fcc symmetry and multiply twinned decahedral particles. The narrow size distribution centers around 12 Angstrom. The near-neighbor distance is found to be 0.9% smaller than for the bulk PtRu. XANES data provide evidence for the metallic character of the Pt component. The silica-supported colloid particles oxidize rapidly in air to form an amorphous state but can be rereduced in hydrogen at room temperature. An "open-slit" XRD technique was used to study the particle growth in H-2 and He. Particles annealed to 700 degrees C in He show a twinned fcc symmetry with an average size of 23 Angstrom. The CO oxidation of this catalyst was studied by in situ XRD. At 280 degrees C surface oxide species are formed that slowly coalesce to RuO2 particles. After rereduction the catalyst consists of a pure hcp ruthenium phase and larger alloy particles enriched in platinum.