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Heterogeneous catalysts that contain bimetallic nanoparticles may undergo segregation of the metals, driven by oxidizing and reducing environments. The structure and composition of core-shell Rh_0.5Pd_0.5 and Pt_0.5Pd_0.5 nanoparticle catalysts were studied in situ, during oxidizing, reducing, and catalytic reactions involving NO, O₂, CO, and H₂ by x-ray photoelectron spectroscopy at near-ambient pressure. The Rh_0.5Pd_0.5 nanoparticles underwent dramatic and reversible changes in composition and chemical state in response to oxidizing or reducing conditions. In contrast, no substantial segregation of Pd or Pt atoms was found in Pt_0.5Pd_0.5 nanoparticles. The different behaviors in restructuring and chemical response of Rh_0.5Pd_0.5 and Pt_0.5Pd_0.5 nanoparticle catalysts under the same reaction conditions illustrates the flexibility and tunability of the structure of bimetallic nanoparticle catalysts during catalytic …