Carbon supported Au-Pt catalysts with different bimetallic compositions were prepared by water-in-oil (w/o) micro-emulsions. Carbon Vulcan XC-72R was added during the synthesis of particles in order to obtain good dispersion. Structural characterization was performed using XRD (X-ray diffraction) at wide angles, WAXS (wide-angle X-ray scattering) which showed that Pt-Au particles exhibited alloy properties in samples with high gold-content, and a segregation effect for those with low gold-content. Electrochemical characterization allowed estimation of the surface composition of Pt-Au alloys. These experiments have been confirmed by XRD data. Moreover, HRTEM (high resolution transmission electron microscopy) and XEDS (X-ray energy dispersive spectroscopy) characterization further confirmed the electrochemical results and XRD data. CO stripping experiments have shown an increasing bonding energy between CO and platinum with the gold content in the nanoalloys.