Thermogravimetric technique and boat experiments were used to study the chlorination of MgO and its reactivity with respect to Cl2 + air, Cl2 + N2, and Cl2 + CO gas mixtures at temperatures lower than 1000°C. Oxychlorination of MgO occurs at temperatures higher than that of its carbochlorination. Effects of experimental parameters such as gas flow rate, temperature, and partial pressure of the carbochlorinating gas mixture on the reaction rate were examined. At 550 °C, the apparent reaction orders with respect to Cl2 + CO, Cl2, and CO were 2.37, 1.47, and 0.89, respectively. At this temperature, the maximum reaction rate was obtained using a Cl2 + CO gas mixture having a Cl2/CO molar ratio equal to about 0.6. The apparent activation energy of carbochlorination of MgO was calculated as 49 kJ/mol between 425 °C and 600 °C.