AbstractTo identify the reaction products and process in the reaction of T2–CO 1:1 mixed system, infrared adsorption spectroscopy was applied. From the results of spectral measurements of the reaction products, aldehydes (RCTO), alcohol (ROT) and carboxylic acids (RCOOT) were found in solid phase and tritiated water and methane in gas phase. From rapid resume of absorption band intensities of CO and CO2 following intentional extraction of the sample gas, it was revealed that ‘quasi-equilibrium states’ existed by balancing consumption and reproduction of CO and CO2. The ratio −ΔCO/+ΔCO2 was much higher than that reported for the CO transformation by α irradiation and in proportion to tritium concentration in the gas phase. Tritium was accumulated in the condensed reaction products and estimated to move into the condensed phase from the gas phase about 5% per day without any hygroscopic substance. The solid products of this system, which remained for a long time, would be carboxylic acids.