Radioactive iodine gas is a problematic species in multiple nuclear energy–related applications. Therefore, it is highly desirable to develop an adsorbent that has a high capacity for iodine. In this investigation, the iodine adsorption capacity of high-purity magnesium oxide was investigated as a function of the calcination conditions. Differences in the magnesia substrates were characterized by scanning electron microscopy and X-ray diffraction, and the iodine adsorption capacity was determined using thermogravimetric analysis. The results indicate that the calcination temperature and time have a significant impact on the adsorption capacity, with longer times and higher temperatures having a negative impact. However, under the optimal calcination conditions identified in this study (550°C for 20 min), the high-purity magnesia was found to have an adsorption capacity >300 mg of iodine per gram of sorbent. This suggests that magnesia holds promise for nuclear applications where iodine gas adsorption would be beneficial.