A study about the growth mechanism of ZnO nanowires electrodeposited from the reduction of dissolved molecular oxygen in zinc chloride solutions was reported. It was found that the ZnCl2 concentration is a major parameter to control the dimensions of ZnO nanowires. Mean nanowire diameters froth 25 to 80 nm were obtained by only modifying the ZnCl2 concentration. No significant increase of the nanowire diameter was observed for the charge densities analyzed (2-70 C/cm(2)), suggesting that the lateral growth took place only during the first growth stages. The internal structure of ZnO favored the anisotropic growth along the [000 1] direction of ZnO crystals. During the first growth stage, pyramidal crystals grew from the ZnO nanocrystals of the ZnO buffer layer, leading to the formation of rods and then nanowires. The electrochemical conditions used in the paper resulted in a relatively fast electrochemical reduction of O-2 in comparison to the diffusion of Zn2+ to the cathode. Consequently, ZnO was formed only at the tips of the vertical cylindrical crystals, quenching their lateral growth. Transmission Electron Microscopy revealed that the obtained ZnO nanowires were single crystals, irrespective of [ZnCl2], showing the great potential of this approach to obtain ZnO single crystal nanowire arrays with tailored dimensions. (c) 2007 Elsevier B.V. All rights reserved.