[en] Tin oxide nanopowders, with an average size of 5 nm, were prepared by microwave flash synthesis. Flash synthesis was performed in aqueous solutions of tin tetrachloride and hydrochloric acid using a microwave autoclave (RAMO system) specially designed by the authors. Energy dispersive X-ray analysis (EDX), X-ray powder diffraction (XRD), Brunauer-Emmett-Teller (BET) surface area analysis, nitrogen adsorption isotherm analysis, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), Fourier transform infrared (FTIR) spectroscopy, and transmission electron microscopy (TEM), were used to characterize these nanoparticles. Compared with conventional synthesis, nanopowders can be produced in a short period (e.g. 60 s). In addition, high purity and high specific surface area are obtained. These characteristics are fundamental for gas sensing applications. - Graphical abstract: Tin oxide-based gas sensors have been extensively studied in recent years in order to understand and improve their sensing properties to a large variety of gaseous species. As is well known, high specific surface area increase the responses of gas sensors. Microwave-induced thermohydrolysis appears as an efficient way to produce nanoparticles in a very short time, with controlled size (4-5 nm) and high-specific area (160-190 m² g^-1). Pictogram represents our original microwave reactor, the RAMO (French acronym of Reacteur Autoclave Micro-Onde), containing the reactants and submitted to the microwave irradiation (multicolour candy represent obtained material), and a typical TEM image of the as-prepared SnO₂ nanoparticles