Kirchhoff's law of heat radiation, which relates the spectral emissivity and absorptance of a material body, has been demonstrated for a hot gas (CO 2 ) enclosed in a gas cell at a known temperature. The spectral emissivity of a hot gas in an enclosure is difficult to determine directly by experiment. The temperature-dependent effects of the radiative properties of the cell window materials and self-absorption effects of the hot CO 2 gas have to be measured and accounted for before the spectral emissivity can be calculated. The experimental procedure and analytical expressions describing solely the net radiative properties of the enclosed gas are thus verified by demonstrating that Kirchhoff's radiation law holds.