Bimetallic catalysts can undergo segregation or redistribution of the metals driven by oxidizing andreducing environments. Bragg coherent diffraction imaging (BCDI) was used to relate displacement fieldsto compositional distributions in crystalline Pt-Rh alloy nanoparticles. Three-dimensional images ofinternal composition showed that the radial distribution of compositions reverses partially between thesurface shell and the core when gas flow changes between O2 and H2. Our observation suggests that theelemental segregation of nanoparticle catalysts should be highly active during heterogeneous catalysis andcan be a controlling factor in synthesis of electrocatalysts. In addition, our study exemplifies applications ofBCDI for in situ 3D imaging of internal equilibrium compositions in other bimetallic alloy nanoparticles.