A high-temperature zone formed during the atmospheric oxidation of carbon monoxide on the external surface of a radial-flow reactor, which was a hollow cylinder of reticulated alumina (32 mm o.d. 25 mm i.d. and 102 mm high) impregnated with Pd. The reactor was placed horizontally inside a vessel with a sapphire window and a set of gold-plated mirrors enabled continuous monitoring of the entire cylindrical reactor surface temperature by an infrared camera. The high-temperature region Formed close to the extinction of the uniformly ignited state and was separated by a sharp temperature front from the surrounding cold region. The temperature difference between the cold and hot regions was up to 150 degreesC for a feed containing 6 vol% carbon monoxide and 70 vol% O-2. The size of the hot zone, which was of the order of a few cm, decreased upon cooling of the vessel. The slow motion of the temperature fronts (breathing) caused complex oscillations in the size of the hot zone and the overall reaction rate.