Indium tin oxide (ITO) thin layers were deposited onto glass substrates by RF magnetron sputtering using different pressures. Subsequently, the films were annealed in a reducing atmosphere at 500 degrees C for 30 min. Electrical properties were measured by Hall Effect analysis and four-point probe measurements. Optical properties were determined by UV-Vis spectrophotometery. Film structures and compositions were analyzed by X-ray diffractometry and X-ray photoelectron spectroscopy, respectively. The effect of sputter pressure and additional anneals was investigated. The results revealed that the lowest resistivity of 1.69 x 10(-4) Omega cm was achieved at low pressure (1.2 Pa) and the highest transmittance of similar to 90% was obtained after a second anneal. However, the second anneal decreased the mobility and the conductivity especially for high sputtering pressures. This study also describes the effect of Sn defect clustering on electrical properties of the ITO films. (C) 2009 Elsevier B.V. All rights reserved.