A rapid, clean, and continuous hydrothermal route to the synthesis of ca. 14 nm indium oxide (In2O3) nanoparticles using a superheated water flow at 400 degrees C and 24.1 MPa as a crystallizing medium and reagent is described. Powder X-ray diffraction (XRD) of the particles revealed that they were highly crystalline despite their very short time under hydrothermal flow conditions. Gas sensing substrates were prepared from an In2O3 suspension via drop-coating, and their gas sensing properties were tested for response to butane, ethanol, CO, ammonia, and NO2 gases. The sensors showed excellent selectivity toward ethanol, giving a response of 18-20 ppm.