Crystalline WO3 nanoparticles are employed in the development of flexible electrochromic (EC) devices. The nanoparticles are synthesized at high-density with a hot-wire chemical vapor deposition process where the hot filament provides the source of the tungsten metal. Polyethylene terephthalate coated with indium tin oxide is employed as a transparent flexible substrate. A simple electrophoresis technique is employed to deposit the WO3 nanoparticles on the polymer, resulting in a uniform thin film. The EC performance is optimized for WO3 particles that were baked at similar to 300 degrees C for 2 h prior to electrode fabrication. The transmittance is modulated between similar to 94% and similar to 28% without degradation for 100 cycles. (C) 2009 Elsevier B.V. All rights reserved.