The understanding of the surface properties of binary or ternary Pt-alloy nanoparticles is essential for manipulating their electrocatalytic properties. Carbon-supported binary or ternary Pt-alloy nanoparticles synthesized by reduction-encapsulation chemical methods have recently been shown to exhibit high electrocatalytic activity towards oxygen reduction reaction and methanol oxidation reaction. This paper reports findings of an FTIR investigation of CO adsorption on silica-supported PtFe, PtVFe, and PtNiFe nanoparticles (2-5 nm) with controlled multimetallic composition. The detection of CO stretching frequencies different from the monometallic counterparts has substantiated the surface bimetallic or trimetallic alloy properties. New insights into the correlation between alloy composition and surface binding properties are discussed, as supported by a computational modeling results. The findings have interesting implications to the design of binary or ternary Pt-alloy nanoparticles as highly-active electrocatalysts.