Plasma-enhanced chemical vapor deposition has been used to form electrochromic tungsten oxide thin films from gas mixtures of WF6, O-2, and H-2. The chemistry of this system was investigated using optical emission spectroscopy. The plasma composition and deposition rate were examined as a function of operating conditions. Growth rates were maximized when the atomic fluorine density was attenuated and the atomic oxygen density was saturated. With the exception of hydrogen scavenging of fluorine radicals, the major reaction pathway was simply dissociation, as the densities of the atomic species were proportional to the initial reagent composition. An apparatus was built to evaluate electrochemical performance in direct registry with optical transmission. This characterization technique was used to demonstrate that ion bombardment has a dramatic impact on electrochromic performance. Ion bombardment was found to increase film density, as inferred from measurements of the refractive index. Efficient hydrogen intercalation was observed only in those films whose refractive index at 400 nm were less than 2.1. Finally, it was shown that a tungsten oxide thickness of similar to600 nm was optimal with respect to contrast ratio. (C) 2003 American Vacuum Society.