Electrical and structural properties of indium-tin-oxide (ITO) films with various Sn concentrations deposited under a high oxygen partial pressure were examined to find out how the excess oxygen behaves as a scattering and trapping center of electron carriers and to show evidence that excess oxygen is weakly bound with Sn4+ ions on In3+ sites with single-charge Sn-In(.). ITO films with Sn concentrations of 1-25 wt % were deposited on glass substrates by rf sputtering under various oxygen concentrations in the discharge gas (0.3%-100%); the substrate temperature was kept at 773 K during deposition. With increasing O-2 flow ratio in the discharge gas from 0.3% to 100%, the Hall mobility and the carrier density were found to decrease from similar to 40 to similar to 20 cm(2) V-1 s(-1) and from similar to5.0 X 10(20) to similar to2.0 X 10(18) cm(-3), respectively. The dependence of the slope of mobility-temperature curve on the O-2 flow ratio and Sn concentration has been examined. For high Sn concentration, the slope changes from negative to positive as the O-2 flow rate increases. The slope of the curve for a film with Sn concentration of 8 wt% deposited at an O-2 flow ratio of 100% changes from positive to negative after postannealing at 473 K for 30 min. in N-2. These results suggest that the oxygen attaches weakly to Sn4+ and that the oxygen plays a role in the scattering and trapping center. It is also found that oxygen ions mainly exist not in interstitial quasianion sites but in the grain boundaries because of the constant lattice parameter after annealing.