Abstract Cu-Pt nanopowders were prepared by sonoelectrochemistry followed by a displacement reaction. The first method provides pure copper particles. In the second method the surface copper atoms are replaced by platinum atoms. The influence of dissolved oxygen during the conversion reaction was also studied. Nanoparticles (NPs) were observed by transmission electron microscopy (TEM) and their size distribution was determined by centrifugal liquid sedimentation (CLS). The mean diameter of isolated particles was found to be around 8 nm. Their composition was studied by energy dispersive X-ray spectroscopy (EDXS) and X-Ray photoelectron spectrometry (XPS). These analyses and X-ray diffraction (XRD) patterns showed that the particle shell is a solid solution of Cu and Pt. The shell composition is heterogeneous with a richer Pt percentage on its surface.