Coprecipitation may be a significant process in controlling radionuclide release during spent fuel dissolution/reprecipitation in a breached geological disposal site by water. The objective of this work was to study the coprecipitation of thorium and lanthanum with UO2+x(s) as host phase under reducing conditions (pure N2 and Eh=−300 mV/SHE) as a function of pH and [Th]/[U+Th] or [La]/[U+La] initial concentration ratios. At pH values between 3 to 9, the final concentrations of dissolved Th and La were found to be considerably lower than expected if pure phases, Th(OH)4(s) and La(OH)3(s) would have been formed. Coprecipitation phenomena and formation of solid solutions ThyU1-yO2+x'(s) and (U,La)O2+x'(s) are suggested to be responsible for the lower solubility.