We report on the alloying of epitaxial Co/Pt core-shell nanoparticles using transmission electron microscopy (TEM) and electron diffraction. In as-deposited nanoparticles followed by in situ annealing at 823 K for 10.8 ks, high-angle annular dark-field (HAADF) imaging by scanning TEM (STEM) clearly revealed formation of Coshell/Pt-core structures due to the large atomic number (Z) difference between Co (Z=27) and Pt (Z= 78). We identified a formation of locally ordered areas of the L1(0) ordered phase at the core of the nanoparticles. After ex situ annealing at 873 K for 0.6 ks, some of the ordered areas showed complicated contrasts in the HAADF-STEM images. Based on image simulations, we found that these atypical contrasts arise from the stacking of two orthogonal variants of the L1(0) phase in the electron beam direction. Furthermore, the simulation showed that image contrast strongly reflects the structure of the variant located closer to the beam entrance rather than to the bottom side. Solid solution phase was formed by further annealing at 873 K for 3.6 ks, while high-density (111) stacking faults were observed inside the Co-Pt alloy nanoparticles. Magnetic coercivity remained at values as low as similar to 15.9 kA/m at 300K, irrespective of the formation of local L1(0) ordered areas and/or a high-density stacking faults. (C) 2012 Elsevier B.V. All rights reserved.