Magnesium oxide (MgO), or magnesia, is a white hygroscopic solid mineral that occurs naturally as periclase and is a source of magnesium. It has an empirical formula of MgO and can be used extensively in the soil and groundwater remediation, wastewater treatment, drinking water treatment, air emissions treatment, and waste treatment industries for its acid buffering capacity and related effectiveness in stabilizing dissolved heavy metal species.

Granular MgO is often blended into metals-contaminated soil or waste material, which is also commonly of a low (acidic) Ph, in order to drive the Ph into the 8–10 range where most metals are at their lowest solubilities.