Abstract <p>The diffuse reflection spectra of micro-, submicro- and nanopowders of ZnO after irradiation with solar electromagnetic radiation were studied. High-purity ZnO powders purchased from Aladdin Chemistry were used. Average particle sizes of the studied powders were from 800 to 3000 nm for micropowders, from 100 to 300 nm for submicropowders, and 20–50 nm for nanopowders. Irradiation of the powders under study with solar electromagnetic radiation was carried out for 2, 5, 10, and 15 h. The research results showed that the reflectivity of the surface of zinc oxide micropowders in the wavelength range from 200 to 2000 nm is higher than that of submicro- and nanopowders. The contribution to the formation of the integral absorption band responsible for the degradation of the optical properties of ZnO micro- and nanopowders is made by induced defects of the cationic sublattice and submicropowders, induced defects of the anionic sublattice, and acceptor-donor pairs. Approximately the same intensity of absorption bands of defects in ZnO submicropowders explains the small change in the integral absorption coefficient of solar radiation for this type of powder. This explains the higher radiation resistance of zinc oxide submicropowders to the action of solar spectrum quanta under the same irradiation conditions.</p>