Natural chromite minerals form extended solid solutions with binary spinels of FeCr2O4, MgCr2O4, FeAl2O4, and MgAl2O4. The decomposition of natural spinels strongly depends upon the chemical potential imposed in the forms of temperature, pressure, and pH difference in aqueous media (e.g., during natural weathering). In this investigation, we studied the thermal decomposition behavior of South African chromite ores in order to relate the influence of oxygen potential with the likely product phases formed. The decomposition is also a generic step in the understanding of the formation of sodium chromate during soda-ash roasting and the reduction of chromite ores for ferrochrome alloy making. The phase equilibria in South African chromite minerals were investigated by isochronal thermal analysis and isothermal heat treatment of chromite mineral in air, argon, and Ar-5 pct H2 atmospheres over a temperature range from 473 to 1473 K. The effects of the oxygen partial pressure and temperature on the phase constituents of the heat-treated product are discussed by referring to the results of X-ray diffraction (XRD), scanning electron microscopy with energy-dispersive X-ray detector (EDX), and electron probe microanalysis (EPMA). The structure of phases formed and the morphology of phase-separated regions in chromite appear to be strongly dependent on the oxygen partial pressure. The mechanism of the decomposition of complex spinel phases is described under the influence of oxygen partial pressure and temperature.