Electrochromic tungsten oxide thin films have been prepared by reactive dc-magnetron sputtering under different deposition conditions. Through the use of scanning electron microscopy, spectroscopic ellipsometry, and electrochromic coloration experiments, the effect of preparation conditions on film morphology and electrochromic properties has been studied. The results of this study are consistent with a previous report which found that tungsten oxide thin films are dominated by a hierarchy of dense columnar regions and less dense void regions. This morphology not only dominates the structure of tungsten oxide thin films but also strongly controls the electrochromic properties. From spectroscopic ellipsometry data and Bruggeman effective medium approximation models presented here, it seems reasonable that a cermet model of absorption can accurately describe the tungsten oxide preparationelectrochromic property relationship.