The differential thermopower (Seebeck effect) depends on the properties of two materials that form a junction. If one of the two materials is chemically sensitive, a thermocouple can constitute a chemical sensor that provides a chemically dependent signal similar to that of a chemically sensitive field-effect transistor (CHEMFET). Thermocouples were fabricated from junctions of thin films of indium oxide (In2O3) and gold (Au) sputtered onto a glass substrate. The device, with the junctions Au-In2O3 and In2O3-Au was heated such that the two junctions were kept at slightly different temperatures. Thus, a thermoelectric voltage (Seebeck effect) was generated at the junctions. When the device was exposed to a continuous flow of pure air or synthetic air with traces of nitric oxide (NO2), the Seebeck voltage was found to he extremely sensitive to the NO2 contamination in the air, as expected from theory. We demonstrate the operation of a new chemical sensor that works due to the modulation of the Fermi energy as a result of chemisorption.