A systematic study of the role of KCl on the electrodeposition of ZnO nanowire arrays from the reduction of oxygen in ZnCl 2 solutions was performed. Besides its role as a supporting electrolyte, other major effects were found. An increase of KCl concentration ([KCl]) considerably decreased the rate of O 2 reduction. The consequent decrease in OHproduction rate resulted in an augmentation of the ZnO deposition efficiency, from a value around 3% for [KCl] ) 5 × 10 -2 M to more than 40% for [KCl] ) 3.4 M. The increase of the deposition efficiency mainly resulted in an enhancement of the longitudinal growth rate. However, high [KCl] (>1 M) also favored the lateral growth of the ZnO nanowires, resulting in diameters as big as 300 nm (in comparison to the diameter of 80 nm obtained for [KCl] < 1 M). The observed effects were discussed in terms of Clion adsorption on the cathode surface. The possible preferential adsorption of the anion on the (0001) ZnO surface was emphasized. Transmission electron microscopy revealed that the ZnO nanowires were single crystals, irrespective of [KCl] in the electrolyte. Thus, playing with the chloride content in the solution is an interesting way to obtain ZnO single-crystal nanowire arrays with tailored dimensions under controlled deposition rates. The influence of the nanowire dimensions on the optical properties was also discussed, showing the interest of this study in the frame of nanostructured solar cells.