The kinetics of oxychlorination of MgO by Cl2 + O2 were studied in the temperature range from 850 °C to 1025 °C, using thermogravimetric analysis (TGA). The effects of Cl2/O2 ratio, gas velocity, temperature, and partial pressure of reactive gases on the reaction rate were investigated. The oxychlorination process was characterized by an apparent activation energy of about 214 kJ/mol. The reaction orders with respect to O2, Cl2, and Cl2 + O2 at 950 °C were about −0.37, 0.98, and 0.65, respectively. Data concerning oxychlorination of MgO, Cr2O3, and MgCr2O4 contained in chromite were compared. The effectiveness of using oxychlorination to extract iron oxides contained in magnesia was demonstrated.