The location of dopant ions and the effect of doping level on the photocatalytic activity have been investigated on Co-doped ZnO nanopowders. A co-precipitation method was used to prepare ZnO nanoparticles of <50 nm in diameter doped with up to 5 at% of Co. The crystal structure of nanoparticles and local atomic arrangements around dopant ions were analysed by X-ray diffraction and extended X-ray absorption fine structure spectroscopy using synchrotron radiation. The results showed the Co ions substituted the Zn ions in ZnO crystal lattice and induced lattice shrinkage. The photocatalytic activity under simulated sunlight irradiation was characterised by the decomposition of Rhodamine B dye molecules, which revealed the successful reduction of photocatalytic activity by Co-doping. (C) 2012 Elsevier B.V. All rights reserved.