CuO nanowires have been synthesized by thermal oxidation of copper foils in oxygen atmosphere. Morphology and microstructure of the nanowires was studied as a function of temperature and annealing time using scanning electron microscopy, energy dispersive X-ray analysis, X-ray diffractogram, X-ray photoelectron spectroscopy, transmission electron microscopy and selected area electron diffraction. Nanowires were found to grow preferentially along [010] direction. Length of nanowires was seen to increase with annealing time and for long times branched structure was observed. A "blue-shift" in comparison to bulk CuO was observed in the band gap as determined from optical absorption spectra. Mechanism of growth of CuO nanowires is discussed. (c) 2005 Elsevier B.V. All rights reserved.