SnO₂ and In₂O₃ nanocrystals were prepared by injecting the corresponding metal oxide sols in solutions of amines in organic solvents at 160 °C, followed by heating at the resulting temperature (80–110 °C) for 3 h. The obtained nanoparticles were extracted by centrifugation, washed and heat-treated at 500 °C for final purification and eventual completion of the crystallization process. The preparation process and the final products were characterized by X-ray diffraction, Raman and Fourier transform infrared spectroscopy and transmission electron microscopy. Very small nanoparticles with a mean size of about 6 nm were obtained, generally constituted by single crystals, chemically pure and defect free. The nanocrystals could be suspended in various solvents, and then deposited by dropping onto alumina substrates with pre-deposited heaters and electrodes. The resulting gas-sensing devices were tested to such gases as acetone and ethanol, and we observed that tin oxide has a higher sensitivity towards acetone while indium oxide has a higher sensitivity towards ethanol. Moreover, in the case of indium oxide there is a maximum in the response around 300 °C while for tin oxide the response increases with the operating temperature.