This work presents a comprehensive investigation on the effect of small amounts of copper or manganese on the catalytic and photocatalytic activities of ZnO powders. The observed effects are interpreted based on the theory of chemisorption, the physics of semiconductors and the chemistry of non-stoichiometric oxides. It turns out that the nature of the investigated processes is determined mainly by both, the influence of admixtures on the electronic band structure parameters and the alteration of the sorption ability of the semiconductor surface. (C) 2010 Elsevier B.V. All rights reserved.