Decomposition of the organometallic precursor [In(η5-C5H5)] in toluene in the presence of methanol (8 vol.-%) at room temperature leads to the immediate formation of aggregates of indium nanoparticles of 15 ± 2 nm mean diameter. The aggregates are roughly spherical with a mean size of 400 ± 40 nm. The particles were characterized by means of transmission electron and high-resolution transmission electron microscopies (TEM and HRTEM), and X-ray diffraction (XRD) studies indicate that the powder consists of the tetragonal phase of indium. The thermal oxidation in air of these nanoparticles yields well-crystallized nanoparticles of In2O3 with unchanged morphology (aggregates of nanoparticles of 16.6 ± 2 nm mean diameter with aggregate mean size of 400 ± 40 nm) and without any sign of coalescence. XRD pattern shows that the powder consists of the cubic phase of In2O3. The electrical conductivity measurements demonstrate that this material is highly sensitive to an oxidizing gas such as nitrogen dioxide and barely sensitive to a reducing gas such as carbon monoxide. Its association with SnO2-based sensors allows the selective detection of carbon monoxide (30 ppm) and sub-ppm amounts of nitrogen dioxide (400 ppb) in a mixture at 21 °C and at a relative humidity of 60 %.