Abstract—The present work is devoted to constructing a method for calculating the solubility curves of molybdenum and ruthenium in the solid state in the binary Mo–Ru system with consideration of nanoscale effects. The approach is based on the thermodynamics of phase equilibria, taking into account the surface phenomena within the framework of the Gibbs thermodynamic method of separating surfaces. The surface tension is chosen as such a separating surface. The calculations of solubility were performed taking into account the dimensional dependences of the individual characteristics of metals and the parameters of interparticle interaction in the phases. A good agreement of the obtained results with the available experimental data for the macroscopic case is observed.