Indium-tin-oxide (ITO) thin films have been prepared by DC-magnetron reactive sputtering in an vertical Inline sputtering system. The oxygen content of the ITO films was changed by variation of the sputtering gas composition. All other deposition parameters were kept constant. The transmission and the reflection of ITO films were measured, also the film thickness and the sheet resistance. For ITO thin films a sheet resistance of 23 Omega/sq., a maximum transmission of about 89% and a specific resistivity of 216 mu Omega cm (deposited at RT, 6 min annealed in vacuum at 200 degrees C) could be achieved. The complex dielectric functions were derived from measured data. By applying the Drude theory for free carriers the plasma frequency and the collision frequency could be calculated. The energy gap between the valence band and the conduction band could be evaluated from optical data, too, by using a model proposed by Forouhi and Bloomer. The assumption of parabolic band edges was confirmed by comparing the results with predictions of the Burstein-Moss-Shift-theory. A figure of merit, according to the definition given by Haacke, is presented for all samples investigated.