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Abstract: The electrochromic properties of V2O5− z (vanadium oxide) thin films sputtered onto 60 Ω/□ flexible PET (polyethylene terephthalate)/ITO (indium tin oxide) substrates at various oxygen flow rates were investigated. The metallic vanadium (V) target, sputtered by radio frequency (r.f.) power with argon gases, and reacted with oxygen gases in room temperature (23 °C), was proven to provide extraordinary electrochromic properties. Cyclic voltammetry measurement demonstrated that only low driving voltages from −0.6 V to 1 V were needed to provide reversible Li+ ion insertion and extraction to and from a working electrode. The working electrode was part of a PET/ITO/V2O5− z flexible electrochromic device (FECD) in a 0.1 M LiClO4–PC (propylene carbonate) electrolyte with a Pt counter electrode and a saturated calomel reference electrode. Cyclic voltammetric (CV) analyses and ultraviolet (UV)–visible spectroscopy measured the electrochromic thin films of a-V2O5− z sputtered onto flexible PET/ITO substrate. An oxygen flow rate of 2 sccm with an oxygen deficiency z of 0.169, an insertion coefficient x of 0.097 and ratio of 0.992 can offer excellent light modulation of up to 43.3% of transmittance variance, optical density change of 0.38 and color efficiency of 102.5 cm2/C at a wavelength of 400 nm for 200 cycles of Li+ insertion and extraction. [Copyright &y& Elsevier]