We performed a comprehensive study on the electrochromism in TiO 2 thin films made by reactive DC magnetron sputtering and elucidated the roles of sputter gas pressure p, O 2 /Ar gas ratio γ and substrate temperature τ s . Good mid-luminous optical modulation-taken to be ~50 % in ~200-nm-thick films-was obtained under charge exchange in a Li + electrolyte for p > 15 mTorr and τ s < 100 °C, whereas γ was less important. The deposition rate dropped for increasing p, and hence p ≈ 15 mTorr was optimal. These films were X-ray amorphous and contained some water. The coloration efficiency η was ~25 cm 2 /C, which exceeds data on η in most prior studies on sputter deposited TiO 2 and verifies that such films can display the same values of η as those of TiO 2 films made by several chemical techniques.