We show how a configurational lattice dynamics technique, in which the free energy of a number of configurations is determined directly by means of a fully dynamic structural minimization, can be used to calculate thermodynamic properties of solid solutions and phase diagrams. No assumptions are made as to the nature of the solution and both configurational and vibrational entropy contributions are determined directly. Only a small number of configurations are required. We illustrate the method using MnO/MgO, for which our results support the recent experiments of Wood, Hackler, and Dobson [Contrib. Mineral. Petrol. 115, 438 (1994)] who, in contrast to previous workers, suggest the formation of a complete solid solution at temperatures only above 1100 K.