Thin films with nominal molar compositions ranging from 98% In2O3−2% SnO2 to 2% In2O3−98% SnO2 were prepared by mixing solutions of Sn(II) 2-ethylhexanoate in butanol with In2O3 sols prepared by a chemical complexation-based sol−gel process. The starting solutions were characterized by Fourier transform infrared spectroscopy, while the thin films, prepared by spin coating on silicon substrates and subsequent heat treatments up to 500 or 800 °C, were characterized by X-ray diffraction, field emission scanning electron microscopy, Auger spectroscopy, Fourier transform infrared spectroscopy, high-resolution transmission electron microscopy, and electron energy loss spectroscopy. It was concluded that the main factor governing the film formation is the lack of cross-linking reactions between the Sn and In species in the starting solutions. This may result in phase separation already during the spinning …