A Monte Carlo Exchange technique is used to study the thermodynamic properties of MgO-MnO nanoparticles ranging in size from 1728 to 219:52 ions. The solubility of Mg2+ is much greater in MnO than the reverse, reflecting the difference in size between the two cations. The solubility, for a given temperature, diminishes with nanoparticle size. As the Mn concentration is progressively increased the Mn2+ ions occupy the corners, edges and then surface sites of the nanoparticle before entering subsurface layers. We do not observe any pronounced ordering of the cations within the body of the nanoparticles themselves. The enthalpies of forming ternary nanoparticles from particles of MgO and MnO of the same size vary with the size of the nanoparticle and become more positive for a given concentration as the particle size increases. Free energies of mixing of the two end-member nanoparticles have been determined using the semigrand ensemble. The consolute temperature (the temperature above which there is complete miscibility) increases non-linearly with the size of the nanoparticle by approximately 70% over the size range considered.