The electrocatalytic oxidation of methanol on carbon-supported bimetallic Pt-Os nanoparticle catalysts has been investigated. The as-prepared Pt-Os nanoparticles, which are single-phase disordered structures and well dispersed on the surface of carbon with a narrow size distribution, exhibit, respectively, significantly enhanced and slightly enhanced activities for methanol oxidation when compared to commercial Pt/C and Pt-Ru (1:1)/C catalysts. The maximum activity of our Pt-Os based catalysts for methanol oxidation was at a Pt/Os atomic ratio of 3:1. Such an enhanced activity can be rationalized by a combination of a bifunctional mechanism and an electronic effect.