SnO2 powder showing small particle size has been synthesized by means of pyrolysis of an aerosol produced by ultrahigh-frequency spraying of an aqueous tin sulfate solution. Microstructural characterizations (SEM, TEM, x-ray diffraction) were performed on as-received samples, and on pressed and annealed samples. A microstructural hierarchy was observed : an agglomerate of polycrystalline grains (mean size : 100 nm) themselves formed with clusters of crystallite (mean size : 9 nm). Hence two kinds of crystallite boundaries are to be considered. By annealing at 800, 1000, and 1100-degrees-C a change of the microstructural hierarchy occurs. DC conductance measurements were carried out from 50-500-degrees-C under pure air (G = G0), and air mixed with 80 ppm ethanol or with 300 ppm CO. In presence of reducing gases in air, a marked maximum occurs on G = f(T) curves. It has been established by impedance data that the total resistance of the samples is mainly due to the resistance of the grain boundaries. In addition the capacitance value of the ceramic in air is discussed as well as its large change (x60) which occurs under air/ethanol. This effect is discussed in terms of grain boundary phenomena.