[en] The fission fragment radiolysis of CO₂ and the CO₂-C₃H₈, CO₂-NO₂ systems is studied in comparison with γ radiolysis. Two kinds of back reactions (re-oxidation of carbon monoxide to carbon dioxide) are operative in the fission fragment radiolysis. One (a rapid back reaction) is the unidentified ionic chain reaction which proceeds at low CO concentrations, and the other (a slow back reaction) is assigned to a radical reaction (CO + O + CO₂→2CO₂). This slow back reaction is a characteristic of fission fragment radiolysis. The analysis of results gives an upper limit of the radius of fission fragment in the reaction system, rp<1.04 μg cm^-2. (auth)