Light Burnt magnesium oxide (MgO), also known as magnesia, has a variety of industrial applications, including as the basic ingredient of magnesium oxychloride (MOC) cement. However, it is found that the quality, reactivity in particular, of Light Burnt MgO could vary so much that the workability and performance of magnesia-based final products can be significantly affected. Consequently, how to measure the reactivity of MgO would be the key for the further development and success of industrial applications of magnesia. To solve the problem, a simple, effective and yet economical test method for evaluating the reactivity of MgO is developed in this study, which utilizes the time required for neutralization to take place between MgO particles and an acidic solution. The experimental results show that the solubility of the major salts produced during the solid-solution reactions would fundamentally alter the reaction mechanism and thus the speed of the neutralization process. Moreover, the effects of MgO reactivity on the properties of MOC cement are assessed with a series of cement pastes using different MgO batches of various qualities. Finally, a classification of Light Burnt MgO with specific reactivity ranges for different applications is recommended for end users. © 2008 The American Ceramic Society.