Mixed actinide(III,IV) oxalates of the general formula M2.2UIV1.8AnIII0.2(C2O4)5·nH2O (An = Pu or Am and M = H3O+ and N2H5+) have been quantitatively precipitated by oxalic precipitation in nitric acid medium (yield >99%). Thorough multiscale structural characterization using XRD and XAS measurements confirmed the existence of mixed actinide oxalate solid solutions. The XANES analysis confirmed that the oxidation states of the metallic cations, tetravalent for uranium and trivalent for plutonium and americium, are maintained during the precipitation step. EXAFS measurements show that the local environments around U+IV, Pu+III and Am+III are comparable, and the actinides are surrounded by ten oxygen atoms from five bidentate oxalate anions. The mean metal–oxygen distances obtained by XAS measurements are in agreement with those calculated from XRD lattice parameters.