We report the effect of deposition conditions on the intrinsic color and photochromic properties of amorphous MoO3 thin films (a-films) deposited by R. F. unbalanced magnetron sputtering. Optical transmission spectroscopy was used to measure optical properties of the films. The conversion between Mo6+ and Mo5+ for as-deposited and UV irradiated films was characterized using XPS. Raman spectroscopy was used to confirm that the results of XPS were consistent with the bulk of the films. It is shown that absorption coefficient of as-deposited films increases with Mo5+ content. The temporal evolution of absorption coefficients for all films under UV light irradiation is measured using optical transmission spectroscopy. The largest change in absorption was observed for the film with the highest initial Mo5+ content. The temporal evolution of absorption coefficients for all the films shows initial fast rise within first minute of irradiation. XPS and Raman results show that for all films Mo5+ content increases as a result of UV irradiation except for the film with the highest initial Mo5+ content, for which the Mo5+ content decreases relative to Mo6+ despite the fact that the absorption of the film continues to rise. Further understanding of this mechanism is important since it will lead to enhanced photochromism and extend the photo-colorability of the films beyond the point at which the conversion of Mo6+ to Mo5+ is saturated. (C) 2013 Elsevier B. V. All rights reserved.