Thermodynamic optimization of phase diagrams and thermodynamic properties of the FeO–Fe2O3–MnO–Mn2O3–TiO2–Ti2O3 system at 1 atm pressure was performed based on the literature data and new phase diagram data in this study. Isothermal phase diagrams of the system at 1250–1600 °C in air atmosphere were experimentally determined for the first time using the equilibration and quenching method. The liquid phase and complex solid solutions like pseudobrookite, ilmenite and spinel were described using the Modified Quasichemical Model and Compound Energy Formalism, respectively. A set of optimized model parameters of all phases was obtained which reproduces all reliable thermodynamic data and phase equilibria within 3 mol% in composition and ± 25 °C in temperature. The present thermodynamic results can help the prediction of complex thermodynamic data and phase equilibria of the Fe–Mn–Ti–O system at 1 atm total pressure from 25 °C to above the liquidus temperatures over the entire range of composition and pO2 from metallic saturation to 1 atm.