The present work explores the thermodynamic stability of multicomponent multiphase polycrystalline metal alloys using a theoretical approach based on the regular solution model. We extend the definition of ‘phase’ to homogeneous bodies of any dimensionality, which allows grain boundaries to take on phase-like, or complexion, properties, with energy and chemical composition different from grain interiors. Accordingly, we examine the possible attainment of structural stability due to grain boundary segregation within the framework of equilibrium thermodynamics, making use of a Gibbs free energy function. We also derive a dimensionless version of the model for greater generality, and to properly compare the factors determining the thermodynamic stability of polycrystalline structures. Model predictions are summarized with the help of suitably defined stability maps and a few case studies concerning binary and ternary alloys are discussed.