This paper reports an enhanced coloration in amorphous stoichiometric tungsten oxide (a-WO3) thin films prepared by pulsed dc magnetron sputtering of metallic tungsten with oxygen as sputter gas. The films prepared at higher oxygen pressure (5.2x10(-2) mbar) have high coloration efficiency (182 cm(2)/C), large optical modulation (similar to 83%) and faster switching speed (t(c)similar to 32 s at lambda=633 nm). The work function of the virgin and the colored tungsten oxide thin films measured by Kelvin probe are 4.91 and 4.26 eV, respectively. The coloration process conforms to the recent theory based on an "extended site saturation model." (c) 2006 The Electrochemical Society.