Tungsten oxide films were deposited by reactively sputtering a metallic W target in O2Ar atmospheres. Plasma emission spectra were observed as a function of the oxygen content of the chamber inlet gas using an optical multichannel analyser. The gradual loss of W lines as the O2 content was increased to 10% was accompanied by a drastic reduction in deposition rate. Cyclic voltammetry of the films deposited on ITO-coated substrates reveals that films produced under high oxygen conditions have low charge capacities and retain substantial Li upon initial cycling. The photopic coloration efficiency increased with increasing oxygen in the inlet gas. The electrochemical and electrochromic data exhibit breaks in behavior at the 10% O2 break point defined by optical spectroscopy.