Indium tin oxide (ITO) is a transparent conducting oxide used in a variety of optoelectronic applications. In order to optimize the electrical conductivity of ITO thin films it is necessary to determine this property as a function of optimum electron concentration (e.g., doping of In2O3 with Sn). A new software program called CRYSTAL 92 is used to determine the energy band diagrams of In2O3 and In2O3 doped with Sn. Using the curvature of the conduction bands, the effective mass of the electrons is estimated and an empirical relationship is established between the effective mass and free carrier concentration using experimentally observed optical effective mass values. The importance of the varying electron effective mass in the prediction of the electron mobility? and hence the electrical conductivity, is shown here by comparing the published experimental results with the estimated results. The limiting factor in thr electron mobility appears to be either grain boundary scattering or ion impurity scattering. The mobility due to neutral impurity scattering appears to have a negligible effect on the overall mobility unless the neutral impurity concentration reaches extremely high values.