Intermetallic catalysts are of immense interest, but how heterometals diffuse and related interface structure remain unclear when there exists a strong metal-support interaction. Here, we developed a kinetic diffusion–controlled method and synthesized intermetallic Pt ₂ Mo nanocrystals with twin boundaries on mesoporous carbon (Pt ₂ Mo/C). The formation of small-sized twinned intermetallic nanocrystals is associated with the strong Mo-C interaction–induced slow Mo diffusion and the heterogeneity of alloying, which is revealed by an in situ aberration-corrected transmission electron microscope (TEM) at high temperature. The twinned Pt ₂ Mo/C constitutes a promising CO-resistant catalyst for highly selective hydrogenation of nitroarenes. Theoretical calculations and environmental TEM suggest that the weakened CO adsorption over Pt sites of Pt ₂ Mo twin boundaries and their local region endows them with high CO resistance, selectivity, and reusability. The present strategy paves the way for tailoring the interface structure of high–melting point Mo/W-based intermetallic nanocrystals that proved to be important for the industrially viable reactions.