SnO2, TiO2, and In2O3 sols were prepared by a chemically modified sol–gel process. By injecting the sols into a solution of two components at 160 °C in non-solvothermal conditions, differently crystallized products were obtained, which were finally crystallized and purified by heat-treatments at 500 °C. The heat-treatments did not result in remarkable grain growth, as shown by X-ray diffraction and transmission electron microscopy. The mean crystallite size in the heat-treated powders was found to range from 3.5 nm for In2O3 to 8.5 nm for TiO2. Drop-coating of the powders onto alumina substrates allowed the fabrication of gas-sensing devices. The SnO2 and In2O3 based devices were tested to NO2, displaying outstanding responses even at temperatures as low as 100 °C, and fast responses and recovery times.