Reactive magnesium oxide nanoparticles and halogen (Cl-2, Br-2) adducts of these MgO particles were allowed to contact certain bacteria and spore cells. Bacteriological test data, atomic force microscopy (AFM) images, and electron microscopy (TEM) images are provided, which yield insight into the biocidal action of these nanoscale materials. The tests show that these materials are very effective against Gram-positive and Gram-negative bacteria as well as spores. zeta-Potential measurements show an attractive interaction between the MgO nanoparticles and bacteria and spore cells, which is confirmed by confocal microscopy images. The AFM studies illustrate considerable changes in the cell membranes upon treatment, resulting in the death of the cells. TEM micrographs confirm these results and supply additional information about the processes inside the cells. Overall, the results presented illustrate that dry powder nanoparticulate formulations as well as water slurries are effective. It is proposed that abrasiveness, basic character, electrostatic attraction, and oxidizing power (due to the presence of active halogen) combine to promote these biocidal properties.