The effects of groundwater radiolysis on the oxidation and dissolution of UO2 has been studied. The production of a specific radical was maximized by using suitable solution chemistry, and the effect of a particular radical was investigated. Experiments were performed to study the effects of OH, O2, and CO, radicals. An electrochemical cell and a UO2 electrode were designed to do ijri situ electrochemical measurements in gamma fields, 300 -20 Gy/h. During radiolysis, the oxidation of UO2 was monitored by recording the corrosion potential of the UO2 electrode as a function of time. The fastest increase and highest corrosion potentials were observed in solutions favoring the formation of O2 radicals. XPS analysis of the UO2 surface showed that oxidation has proceeded beyond the UO2.33 stage. Computer calculations of water radiolysis showed that steady-state concentrations of the radicals are very low, about 10−9 mol-dm−3, in most cases, and even lower if the reactions with the UO2 surface are included. The radicals are produced at a much faster rate than the observed changes in the corrosion potential, suggesting thereby that another slower reaction is corrosion rate controlling.