Magnesium oxide with high specific surface area (HS-MgO) has been widely used for the sorbent material for hazardous anions from contaminated solutions. Because the magnesium oxide is unstable under ambient conditions, alteration behavior of HS-MgO was investigated by batch alteration experiments as a function of time in electrolyte solutions under ambient and nitrogen atmospheres. The mineralogy of reacted solid was characterized by means of X-ray diffraction, scanning electron microscope and measurements of specific surface area. The solution chemistry of reacted solutions was examined by pH meter and high pressure liquid chromatography for measurement of dissolved Mg concentration. Periclase in HS-MgO almost completely transforms to brucite within 8 hours. The specific surface area decreases from 110m^2/g of unaltered HS-MgO to 60m^2/g by the alteration. The alteration process is characterized by dissolution of periclase and precipitation of brucite from solution. The rate limiting step of the transformation process is the dissolution of periclase. This study indicates that the alteration process must influence on the uptake of hazardous anion by HS-MgO.