The phase constitution during the sintering of pure red mud and red mud–fly ash mixtures was studied in the temperature range of 900°–1250°C. The phases formed at different sintering temperatures were analyzed by X-ray powder diffraction. The phases that are likely to form at equilibrium at any isotherm were predicted using the Gibbs free energy minimization technique and the databases provided in the FactSage software. Although the thermodynamic prediction is in reasonable agreement with the experimental results for the major phases, there is some disagreement regarding the minor phases, especially the more complex phases. The major limitation of the thermodynamic approach presently is the non-availability of thermodynamic data for several complex and multi-component solution phases.