The difference in the adsorption properties of magnesium oxide in two different physical forms-large micron-sized crystals (CM-MgO) and nanoscale crystals (AP-MgO)-is described. The highly energetic surface of the nanoparticles is significantly more active than the bulk surface in adsorption of nonpolar halogens (Cl-2, Br-2, and I-2) and dipole interhalogen molecules (ICI, IBr, and ICI3). The narrocrystalline and microcrystalline MgO adducts with halogens and interhalogens were characterized by thermogravimetric analysis (TGA), UV-vis and Raman spectroscopies, and transmission electron microscopy (TEM). The bonds of the adsorbates were considerably strained upon adsorption on the nanocrystalline MgO without a molecular dissociation. The results presented in the paper explain the enhanced reactivities of the nanoscale MgO, halogen adducts as halogenating agents of organic molecules and their noticeable biocidal activity.