Extremely small PtRu/C nanocatalysts were prepared via a carbonyl route.A thorough in situ reduction X-ray structural characterization of these catalysts was performed.After synthesis and storage under ambient conditions,the diffraction patterns of PtRu/C catalysts were seriously modified,indicating the surface oxide formation.In the reduced state,the particle size is around 2 nm.The observed relative fluctuations of lattice constants are 3％,which is far too large to be explained by a compositional fluctuation.Their origin is attributed to strong but isotropic strains and is related to the alloy formation.The annealing experiments show all the catalysts present an exceptional thermal stability when annealed in inert ambient,especially that of the Pt1Ru1/C catalyst.Besides,it is interesting to note that there is no thermal expansion evidence from the patterns.