AbstractOctahedral Pt3Ni alloy nanoparticles on carbon and alumina supports are prepared using a simple solid‐state chemistry method and tested for CO preferential oxidation (PROX) in H2. Both catalysts exhibit much superior PROX property comparing to spherical Pt3Ni alloy and pure Pt, with high activity and 100 % selectivity between around RT and 100 °C. The improved activity of the octahedral Pt3Ni has been attributed to bifunctional sites mechanism. The excellent selectivity is correlated with the unique surface geometry and electronic structure of the octahedral Pt3Ni.