Molecular dynamics simulations have been employed to study the thermodynamic stability of nanometersized Ag50Cu50 spherical particles with radius in the range between 1 and 10 nm. Four different structural arrangements of Ag and Cu atoms were considered, namely an unmixed phase in which Ag and Cu are separated by a coherent interface, two core-shell systems with Ag shells and Cu cores and vice versa and a perfect random solid solution. All of these cases share the same crystallographic arrangement of atoms but a different chemical order. Free-energy differences between the unmixed phase and the other ones were estimated according to the Bennett’s method. It is shown that free-energy differences scale inversely with the particle radius. The results obtained also provide information on fundamental thermodynamic properties such as surface and interface free energies.