Structural and electrochromic property studies of mixed oxide TaxW1-xO3-x/2 films grown on indium tin oxide coated glass substrates by the reactive pulsed laser deposition in 5.32 Pa oxygen atmospheres are presented in this article. The crystallization temperatures of TaxW1-xO3-x/2 increase with the Ta content in the films: When x=0, TaxW1-xO3-x/2 (i.e., WO3) films crystallized to a monoclinic phase at substrate temperatures as low as 400 degrees C; when x=0.1, TaxW1-xO3-x/2 (i.e., Ta0.1W0.9O2.95) films crystallized mainly to a tetragonal phase at substrate temperatures >= 600 degrees C, and when x=0.3, TaxW1-xO3-x/2 (i.e., Ta0.3W0.7O2.85) films crystallized to a cubic crystal structure at substrate temperatures >= 700 degrees C. As the Ta content increases, the degree of symmetry of crystallized TaxW1-xO3-x/2 films increases resulting in shortening the metal oxygen bond length and forming more rigid structure. Electrochromic properties of amorphous TaxW1-xO3-x/2 films with different Ta contents were evaluated in 0.1M H3PO4 solution. Under H+ intercalation at negative electrical potentials, WO3 (x=0) film changes its color from light pink to midnight blue, Ta0.1W0.9O2.95 (x=0.1) from light pink to cadet blue, and Ta0.3W0.7O2.85 (x=0.3) from light green to light brown green. The colors of all TaxW1-xO3-x/2 films at colored state disappeared immediately after the H+ ions were extracted from the film at position electrical potentials showing excellent reversibility. With similar film thickness the change in optical density between colored state and bleached state for the WO3 is about 2 times higher than that of Ta0.1W0.9O2.95 and about 14 times higher than that of Ta0.3W0.7O2.85. The coloration efficiencies at wavelength lambda=633 nm for WO3, Ta0.1W0.9O2.95, and Ta0.3W0.7O2.85 films are 132, 122, and 65 cm(2) C-1, respectively. The durability of TaxW1-xO3-x/2 films increases with the increase of the Ta content in the acidic environments: in 0.1 M H3PO4 solution the etching rate of WO3 is about 4.6 times higher than that of Ta0.1W0.9O2.95, while Ta0.3W0.7O2.85 has not shown any noticeable degradation after 55 coloring/bleaching cycles (1.5 h). The excellent reversibility of ion intercalation/deintercalation, relatively high coloration efficiency, and improved chemical stability and cyclic durability suggest that Ta0.1W0.9O2.95 and Ta0.3W0.7O2.85 are good candidates as the electrochromic cathodic layer for electrochromic devices, especially for proton conducting liquid-gel-type devices. The results have also demonstrated that addition of one metal oxide (e.g., Ta2O5) into another (e.g. , WO3) is an effective way to alter the electrochromic properties of the individual constituents and provides convenient method for tailoring electrochromic properties of materials. (c) 2006 American Vacuum Society.