The ability to control the composition and phase properties of bimetallic nanoparticles is critical in exploring catalytic properties. In this paper we present results from a study aimed at determining those properties for carbon-supported gold-platinum (AuPt) catalysts with different bimetallic compositions. The bimetallic nanoparticle catalysts are prepared by a two-phase synthesis protocol employing organic monolayer encapsulation on bimetallic AuPt cores (∼2 nm). The size-controlled nanoparticles are assembled on carbon black support materials with controllable dispersion and metal loading and are further treated by calcination under controlled temperature and atmosphere. The core composition of the bimetallic nanoparticles is determined by direct current plasma-atomic emission spectroscopy. Structural characterization is carried out by X-ray diffraction. The bimetallic nanoparticles were shown to display alloy properties, which is in sharp contrast to the bimetallic miscibility gap known for the bulk counterpart of the bimetallic metals. This finding demonstrates the difference of the physical and chemical properties for nanoscale materials from the bulk crystalline state, revealing important details of the phase properties of the bimetallic nanoparticle catalysts and new information for the correlation between the composition and the phase properties at the nanoscale. Implications of our findings to the design and manipulation of the bimetallic nanoparticles for catalytic applications are also discussed.