This paper reports on the deposition conditions and properties of WO, films used as electrochromic layer in a monolithic device on a plastic substrate. The deposition technique employed was roll-to-roll pulsed de magnetron sputtering. The oxide was deposited at room temperature in an argon and oxygen plasma on a transparent conducting ITO layer previously coated on a PET film. The influence of deposition parameters such as purse mode, total pressure and oxygen partial pressure has been investigated, in order to obtain the best compromise between a high deposition rate and adequate electro-optical properties. As expected, the deposition rate increases with decreasing total pressure and oxygen partial pressure. To control the coloring/bleaching behavior, lithium ions from an aprotic electrolyte have been intercalated in films. Films with thickness around 400 nm exhibit a contrast ratio of 9:1. The optical efficiency increases slightly with increasing total pressure and lies between 30-40 cm(2) C-1. Further, the coloring and bleaching time decreases with increasing total pressure, coloring being faster than bleaching. This is consistent with UV-Vis ellipsometric measurements which reveal that the refractive index decreases with increasing total pressure. TEM investigations also confirm that films deposited at low total pressure exhibit a dense structure whereas those deposited at high-pressure exhibit a columnar structure.