The goal of this study has been to investigate the influence of various post-deposition heat treatments on the microstructure, electrical and optical properties of In2O3:Sn (ITO) thin films deposited by electron beam evaporation. We have shown that electron beam evaporated ITO thin films deposited onto substrates kept upto 150 degrees C, have poor electrical properties and low optical transmission in the visible range, due to their amorphous structure. As the microstructure changes from amorphous to poly-crystalline it was observed that the film resistivity decreases and it is simultaneously related to an improvement in the optical transmission. From comparisons of several annealing processes it has been observed that oxygen plays an important role in doping as well as the presence of Sn in the target material. Furthermore we have shown that high quality ITO thin films can be reproducibly prepared with optical transmission being enhanced by an annealing in air and the electrical characteristics being improved by a further annealing in a reducing atmosphere. Superior electrical and optical properties could be correlated with annealed films that exhibited a cubic bixbyte structure and large crystallite dimensions larger than 50 nm. (C) 2010 Elsevier Ltd. All rights reserved.