Ostwald ripening has been broadly studied because it plays a determinant role in the evolution of cluster size during both chemical and physical synthesis of nanoparticles. This thermoactivated process causes large particles to grow, drawing material from the smaller particles, which shrink. However, this phenomenon becomes more complex when considering the coarsening of metallic alloy clusters. The present experimental and theoretical investigations show that the relative composition of CoPt nanoparticles can be strongly modified during high temperature annealing and displays a size-dependent behavior. This compositional change originates from the higher evaporation rate of Co atoms from the nanoparticles. More importantly, this effect is expected in all alloy clusters containing species with different mobilities.