Electrochromic molybdenum trioxide thin films were prepared by thermal oxidation in air of electrodeposited molybdenum trisulfide. MoO3 prepared at temperatures ranging from 400 to 550 degrees C were found to be polycrystalline. The best electrochromic properties (coloration efficiency and coloration time) evaluated in 1 M LiClO4/propylene carbonate were observed for films prepared by annealing at 425 degrees C for 10 min. The coloration efficiency (35 cm(2)/C) for these optimum films compare well with those reported in the literature. The intensity of coloration of the film (absorbance at 634 nm) was found to scale linearly with film thickness. The electrochemical reduction of MoO3 involves the transformation of Mo6+ species to Mo5+, evidenced by x-ray photoelectron spectroscopy. The stability of molybdenum trioxide-coated tin oxide electrodes upon cycling and switching between oxidized and reduced states was investigated by cyclic voltammetry, potential-step experiments, and in situ spectroelectrochemistry. The results indicate that the stability of the molybdenum trioxide electrode is improved when it is used in a 0.1 M LiClO4, 14 mM HClO4, 2% H2O/PC solution rather than in a pure, nonaqueous electrolyte such as 0.1 M LiClO4/PC. The improved stability is related to the fact that the chemical disorder induced by cycling the MoO3 electrode in the protic media is much less severe than in the aprotic media, primarily because while Li+ are intercalated in the latter, only protons are intercalated in the aqueous/nonaqueous mixture.