Thin films of Ta0.1W0.90Ox were deposited on indium tin oxide (ITO) coated glass substrates by reactive pulsed laser ablation in O-2 ambient gas at substrate temperatures of 20, 200, 400, and 600 degreesC. X-ray diffraction (XRD) results showed that Ta0.1W0.9Ox films crystallized mainly to a tetragonal phase at a substrate temperature of 600 degreesC, while films deposited at lower substrate temperatures were amorphous. Lattice constants of polycrystalline Ta0.1W0.9Ox film deposited in 5.32 Pa O-2 are almost the same as those of stoichiometry Ta0.1W0.9O2.95 material, which indicates a truly congruent ablation has been achieved by the laser ablation technique. Films deposited in lower O-2 pressures have poor crystallinity and may contain a lot of defects (oxygen vacancies). For all the temperatures investigated, Ta0.1W0.9Ox films appear almost colorless when deposited in 5.32 Pa O-2, while films have colors of light blue, blue and black when deposited in 2.66, 1.33 and 0.13 Pa O-2, respectively. Optical transmittance for as-deposited Ta0.1W0.9Ox films and the films subjected to H+ intercalation/deintercalation in 0.1 M H3PO4 electrolyte was also measured. H+ ion insertion under an applied electrical potential causes the colors of the Ta0.1W0.9Ox films change from almost colorless (5.32 Pa film) or light blue (2.66 Pa film) to deep blue, while the color contrast between as-deposited and H+ intercalated films is not so significant for the 1.33 and 0.13 Pa films. The H+ ion insertion and extraction processes are fully reversible for the amorphous films, while the optical transmittance of polycrystalline films after the H+ ion extraction process is still low and does not recover to that of as-deposited state. (C) 2004 Published by Elsevier B.V.