Alloy nanostructures have been extensively exploited in both thermal and electrochemical catalysis due to their beneficial “synergetic effects” and being cost-effective. Understandings of the alloy nanostructures including phases, interfaces, and chemical composition are prerequisites for utilizing them as efficient electrocatalysts. Here, we use carbon-supported CuAu nanoparticles as a model catalyst to demonstrate the phase-separation induced variation of electrochemical performance for the CO₂ reduction reaction. Driven by thermal oxidation, the CuO_x phase gradually separates from the original CuAu nanoparticles, and different carbon supports, i.e., graphene vs. carbon nanotube lead to a reversed trend in the selectivity towards CO production. Through detailed structural and chemical analysis, we find the extent of phase separation holds the key to this variation and could be used as an effective method to tune the electrochemical properties of the alloy phase.