Thin films of indium tin oxide (ITO) sputter-deposited by dc-plasma containing deuterium on glass substrate without any heat treatments exhibited gradual lowering in electrical resistivity with increasing the deuterium content [D(2)] in plasma gas by 1% and then demonstrated a jump in resistivity by further increase of [D(2)] than 1%. X-ray photoelectron spectroscopy revealed that hydroxyl-bonded oxygen in ITO grew continuingly with [D(2)]. Deuterium positioned at the interstitial site increased almost quantitatively with increasing [D(2)] Rutherford backscattering spectroscopy showed gradual reduction in the oxygen content of ITO with increasing [D(2)] by 1% and then demonstrated an abrupt increase of the oxygen content with the increase of [D(2)] than 1%. The films with [D(2)] < 1% were oxygen deficient, but those with [D(2)] > 1% were excess of oxygen. The most oxygen deficient film of [D(2)] = 1% was the most conductive. Behavior in the resistivity with [D(2)] looks parallel to that in the oxygen content. A lower resistivity of the films corresponded well to oxygen vacancy rather than hydrogen interstitial. (C) 2011 Elsevier B.V. All rights reserved.