The high-density synthesis of crystalline tungsten oxide nanoparticles employing hot-wire chemical vapor deposition (HWCVD) and enhancement in electrochromic (EC) performance by incorporating these nanoparticles into porous films has been previously reported. Here varying the oxygen concentration during the HWCVD synthesis of these crystalline tungsten oxide (WOx) nanoparticles is examined in order to better understand the mechanism for the improvement in the EC films. Transmission electron microscopy, Raman spectroscopy, X-ray and electron diffraction are used to determine the particle sizes and crystalline phases of the as-synthesized nanostructures. Nanoparticle films are made employing an electrophoresis deposition technique. Cyclic voltammetry of the nanostructured films show higher charge insertion capacities for the nanoparticles synthesized at comparatively lower oxygen concentrations. Consistent with the electrochemical measurements, optical measurements also indicate a higher coloration efficiency (CE) value of similar to 42 cm(2)/C for a nanostructured film made using nanoparticles synthesized at lower oxygen concentration (5%) as compared to the CE value of similar to 24 cm(2)/C for a nanostructured film made using nanoparticles synthesized at higher oxygen concentrations (16%). The CE value of the former is comparable to state-of-the-art amorphous films with the crystalline nanostructures exhibiting significantly improved durability over amorphous films. Notably, the nanoparticle films have been shown to be stable for 3000 cycles in an acidic electrolyte where the amorphous films degrade after only 500 cycles. The optimized EC functional improvements are attributed to a sub-stoichiometric (oxygen deficient) state of WO3. (C) 2007 Elsevier B.V All rights reserved.