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Bimetallic nanoparticles are particles a few nanometres across that contain two metallic elements, either alloyed in one structure or segregated into a core and a shell or into separate domains. Combining two metals is a way of breaking the scaling relations that limit a single metal: the second component changes the electronic structure of the surface through strain and through ligand effects, and can provide a site that one metal alone does not have, so activity and selectivity become adjustable independently. The phenomenon is usually described as synergy, and how far it can be separated from a simple ensemble or dilution effect is a recurring question in the literature ^[1]. Alloy nanoparticles have been reviewed from that point of view, with the emphasis on which geometric arrangement gives the claimed effect ^[2]. The surface is not fixed under reaction conditions: adsorbates restructure the particle and even invert the segregation, so the working surface may differ from the one prepared ^[3]. Selective oxidation over gold-containing bimetallic and trimetallic particles was among the first demonstrations ^[4]. Supported platinum-palladium particles for alkane oxidation are a more recent case where the composition is optimised against the support ^[5]. Segregation under reaction conditions and the difficulty of characterising the surface of a working particle remain the obstacles.