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Abstract: We present a newly developed software framework, MMA‐EoS, that evaluates phase equilibria and thermodynamic properties of multicomponent systems by Gibbs energy minimization, with application to mantle petrology. The code is versatile in terms of the equation‐of‐state and mixing properties and allows for the computation of properties of single phases, solution phases, and multiphase aggregates. Currently, the open program distribution contains equation‐of‐state formulations widely used, that is, Caloric‐Murnaghan, Caloric–Modified‐Tait, and Birch‐Murnaghan–Mie‐Grüneisen‐Debye models, with published databases included. Through its modular design and easily scripted database, MMA‐EoS can readily be extended with new formulations of equations‐of‐state and changes or extensions to thermodynamic data sets. We demonstrate the application of the program by reproducing and comparing physical properties of mantle phases and assemblages with previously published work and experimental data, successively increasing complexity, up to computing phase equilibria of six‐component compositions. Chemically complex systems allow us to trace the budget of minor chemical components in order to explore whether they lead to the formation of new phases or extend stability fields of existing ones. Self‐consistently computed thermophysical properties for a homogeneous mantle and a mechanical mixture of slab lithologies show no discernible differences that require a heterogeneous mantle structure as has been suggested previously. Such examples illustrate how thermodynamics of mantle mineralogy can advance the study of Earth's interior. [ABSTRACT FROM AUTHOR]