The photocatalytic activity of highly c-axis oriented ZnO nanowires' arrays grown on seeded glass substrates from aqueous solutions was investigated against the degradation of stearic acid under UV-A light illumination (365 nm). All chemically grown ZnO samples show very good crystallinity and high transmittance in the visible region. The nanowires' diameter ranges from 35 5 nm to 160 +/- 10 nm depending on the grain size of the seed layer applied, while their length (1-1.2 mu m) is mainly affected from growth time. It is demonstrated that ZnO nanowires' arrays show superior photocatalytic activity compared with the stand-alone seed layers, probably due to their enhanced crystallinity and larger surface-to-volume ratio. (C) 2010 Elsevier B.V. All rights reserved.