VNIIEF have been conducting chemical oxygen-iodine laser (COIL) research since 1982. With successive set-up optimizations, this laser had its power increased by 1986 from 10 mW to lkW and was used as a basis to design a new research set-up of about 4 kW power and about I min continuous operation . COIL is traditionally designed to incoiporate a singlet oxygen generator (SOG), a heat exchanger, a laser volume and a pump out system. The schematic diagram of this laser is shown in fig. I. One of the most important parameters of COIL active medium is its temperature. One may use temperature measurements at various gas flow sections as indications of the gas flow and gas mixing homogeneity, processes kinetics, etc. During operation, alkali-solved hydrogen peroxide cooled down to T=255 is put into SOG. As chlorine gas is bubbled therethrough, singlet oxygen is generated. The oxygen together with water vapor enters the heat exchanger having temperature T= 193 K. While passing through the heat exchanger, the oxygen cools and becomes free of water vapor. The laser volume will have iodine - argon or iodine -nitrogen gas mixtme at T=393 K injected to the singlet oxygen flow. To protect the resonator mirrors, there will be also argon or nitrogen screen made in the laser volume, this being at room temperature. The gas flow has velocity up to I 00 mis in laser volume.