Doped oxide semiconductors,which are widely used as transparent heat-reflectors, have a wider energy gap than the undoped material. This bandgap widening was investigated in 1n203 and 1n203:Sn. Empirical data were extracted for coatings with electron density 1021 cm-3. They are interpreted within an effective-mass-model for n-doped semiconductors well above the Mott critical density. The impurities are ionized and the associated elect-rons occupy the bottom of the conduction band in the form of an electron gas. The model accounts for a Burstein-Moss shift as well as electron-electron and electron-impurity scattering treated in the Random Phase Approximation. Experiments and theory were recon-ciled by assuming a parabolic valence band with an effective mass ti 0.6 m. Earlier work on doped oxide semiconductors are assessed and criticized in the light of the present results.