CuO nanopowders were prepared by precipitation from alkaline solutions and were studied by TPR-CO + O-2, XRD, TGA, TPD-He and XPS. All of the precipitated samples were characterized by excellent catalytic properties toward the low-temperature (LT) oxidation of CO with similar T-50 values of 110 degrees C. In contrast, bulk CuO oxides with sizes greater than 450 nm exhibited no activity at low temperatures. Several monolayers of chemisorbed species, such as water/hydroxyls and carbonate/hydrocarbonates, were typically observed at the surface of the nanopowders. These species were not critical for the LT oxidation of CO, and their preliminary removal did not substantially change the activity of the nanopowders. XPS results indicated a high deficiency of the oxygen sublattice of the CuO1-x (x=0.1-0.15) nanopowders, whereas, for the lattice of bulk CuO, the Cu/O ratio was 1. The highly deficient oxygen sublattice resulted in a disproportionation process, which, in turn, resulted in two observed oxygen forms. An oxygen form with E-b(O1s)= 531.3 eV that is highly reactive toward CO was proposed to be responsible for the high catalytic activity of the CuO nanopowders. Slight differences in the Cu2p shake-up satellite structures were observed between the bulk and nanosized samples, which indicated that the electronic structure in the cationic sublattice had changed. (C) 2012 Elsevier B.V. All rights reserved.