Thin indium tin oxide (ITO) films were deposited by reactive DC magnetron sputtering to study the influence of oxygen and low energy ion bombardment on film growth and film properties. Films were deposited at various oxygen gas flows (0 to 2 seem) as well as negative substrate voltages (0 to -50 V). The film properties were investigated by grazing incidence X-ray reflectometry (GIXR), X-ray diffractometry (GIXRD), AFM and XPS. With increasing oxygen flow film structure and composition change from crystalline metallic In/Sn to Xray-amorphous ITO. Simultaneously the deposition rates decrease from 0.6 to 0.25 nm/s and the film densities increase from 4.3 to 7.1 g/cm(3). The metallic films consist of large grains forming a rough surface. The deposition with higher oxygen flows causes smooth surfaces, grain sizes are not clearly observable. In principle an increasing negative substrate voltage leads to the same film properties but it works like a diminished oxygen flow. Post deposition annealing causes the formation of crystalline ITO films. GIXRD measurements were carried out in situ at 200 degrees C at 10(-5) mbar. Two processes determine the ITO crystallite growth, a fast crystallization of the deposited amorphous indium tin oxide and the diffusion of oxygen into the layer. The formation rate k of the diffusion limited reaction depends on the oxygen flows. With increasing oxygen flow k approximates zero.