This report presents studies performed on high contrast electrochromic devices based on WO3 nanowires. The devices were made in two formats. The first kind was made with vertically oriented nanowires grown on fluorinated tin oxide (FTO) coated glass substrates using a low pressure hot filament CVD process. The devices made with these substrates showed a highly reversible transmission modulation of over 70% with almost 0% transmission in the colored state, at a wavelength of 700 nm. In the second kind of devices, made using dispersed nanowires in a mat-like format, a transmission modulation of over 50% was observed in the same wavelength regime. The bleaching times of electrodes with high densities of nanowire arrays showed a large dependence of bleaching timescales based on the coloration timescales used. Beyond the observed enhancement in the optical transmission contrast characteristics, the coloration and bleaching timescales can be further improved by optimizing the nanowire array characteristics such as their densities and aspect ratios. (C) 2007 Elsevier B.V. All rights reserved.