A critical evaluation and thermodynamic modeling for thermodynamic properties of all oxide phases and phase diagrams in the Mn-O system are presented. Optimized Gibbs energy parameters for the thermodynamic models of the oxide phases were obtained which reproduce all available and reliable experimental data within error limits from 298 K (25 °C) to above the liquidus temperature at compositions covering from MnO to MnO₂, and oxygen partial pressure from 10⁻¹⁵ to 10² (bar). The optimized thermodynamic properties and phase diagrams are believed to be the best estimates presently available. Two spinel phases (α– and β-Mn₃O₄) were modeled using Compound Energy Formalism (CEF) with the use of physically meaningful parameters. Valence states of the spinels are interpreted based on the available thermopower measurement, for which Mn⁴⁺ was considered in the cubic spinel (β-Mn₃O₄). The present Mn₃O₄ spinel solutions can be integrated into a larger spinel solution database, which has been already developed. The database of the model parameters can be used along with a software for Gibbs energy minimization in order to calculate any type of phase diagram sections and thermodynamic properties.