Core-shell nanoparticles are known to form in binary systems using a one-step gas-condensation deposition process where a large, positive enthalpy of mixing provides the driving force for phase separation and a difference in surface energy between component atoms creates a preferential surface phase leading to a core-shell structure. Here, core-shell nanoparticles have been observed in systems with enthalpy as low as -5 kJ mol(-1) and a surface energy difference of 0.5 J m(-2) (Mo-Co). This suggests that surface energy dominates at the nanoscale and can lead to phase separation in nanoparticles. The compositions and size dependence of the core-shell structures are also compared and no core-shell structures are observed below a critical size of 8 nm.