Here we report radiation-induced formation of Ce(III) nanostructures in an in situ liquid cell for the transmission electron microscope (TEM). Small (<5 nm) irregular Ce(OH) 3 nanoparticles were identified as the final products from cerium(III) nitrate solutions of initial pH 5.2. Pourbaix diagrams show that solid Ce(OH) 3 can only exist above pH 10.4, whereas at lower pH values, Ce(III) should remain soluble as aqueous Ce 3+. Reduction of Ce 3+ to zerovalent Ce by aqueous electrons followed by hydrolysis is a plausible catalytic mechanism for generating hydroxide. Numerical simulations support that radiolysis of cerium nitrate solutions may lead to pH increases, in contrast to well-known acidification of pure water. Compared to previous radiolytic synthesis routes in aqueous solution for other metal or metal oxide nanoparticles, based on metal ion reduction, for example, the chemical pathways leading to these Ce(III) nanostructures require an increase in the local pH to alkaline conditions where Ce(OH) 3 can exist. These results extend the range of chemical conditions that can be induced by radiolysis to form oxidized nanostructures from solution.