Oxidation and reduction kinetics of UO2+x, was investigated in CO2/CO gas mixtures at elevated temperatures of 1000 degreesC to 1300 degreesC by monitoring the electrical conductivity relaxation induced by an abrupt change of ambient oxygen partial pressure. By fitting the experimental relaxation data to theoretical equations, the oxidation and reduction kinetics was found to be controlled mostly by the surface reaction step in the ranges of oxygen partial pressure and temperature examined and the surface reaction rate constants were derived therefrom.