The reactivity, or rate of consumption, of magnesium oxide (MgO) powder used in forming magnesium oxychloride (MOC) cement is largely affected by its thermal history and particle size. That, in turn, influences both reaction rate and properties of the reaction products of MOC cement. It is believed that the traditional measurements of properties of MgO powder itself are not able to reflect the actual consumption rate of MgO during chemical reaction, which is the key information needed for the appropriate proportions of starting materials to ensure the formation of Phase 5 crystals for better strength and physical properties. The experimental results show that X-ray diffractograms and phase diagram of a ternary MOC system can be utilized to evaluate the reactivity of MgO, and to optimize the formulation of MOC cement as well. Consequently, the apparent molar ratios of MgO over magnesium chloride (MgCl) are adopted and recommended. The equivalent weight ratios between MgO and MgCl are also given for the convenience of normal applications. © ASCE.