Uranium–cerium oxide solid solutions, U$_{1−x}$ Ce$_x$O$_{2+δ}$·$n$H$_2$O, were prepared through hydrothermal conversion of mixed U(IV)–Ce(III) oxalate precursors, cerium being used as a surrogate for plutonium. Whatever the starting pH, the fluorite-type structure of AnO$_2$ was obtained after heating at 250 °C for 24 h. The initial pH of the reaction media appeared to affect significantly the oxide morphology: for pH ≤ 2, the powder was found to be composed of microspheres, whereas for more alkaline pH values, agglomerates of nanocrystallites were found. Furthermore, a study of the hydrothermal treatment duration (T = 250 °C, pH = 8, t = 1–48 h) showed that fluorite-type mixed dioxides started to form after only 1 h, and then became single phase after 3 h. SEM and TEM/EDS analyses revealed that the cationic distribution narrowed with time to finally form highly homogeneous mixed oxides. Such a preparation route was then applied to various cerium incorporation rates and it was found that the formation of U$_{1−x}$ Ce$_x$O$_{2+δ}$·$n$H$_2$O mixed oxides was possible for 0.1 ≤ x ≤ 0.75. In all the systems investigated, the speciation of uranium and cerium was questioned in both the solid and liquid phases. Thermodynamic calculations and evaluation of the O/M ratio in the final oxides led us to understand the complex redox behaviour of uranium and cerium in solution during hydrothermal processes and to propose a conversion mechanism.