Actinyl ions (AnO(2)(+) and AnO(2)(2+)) are strongly complexed by CO32- in aqueous solution, and tricarbonato complexes of UO22+ and NpO2+, in particular, have been widely studied. Cyclic voltammetry shows that, whereas the Np(V)/Np(VI) couple is reversible in aqueous CO32- solution, the analogous U couple is' irreversible, suggesting significant conformational change on one-electron reduction of [UO2(CO3)(3)](4-). EXAFS spectroscopy has been used to define the coordination geometry of the reduced species and shows that both axial and equatorial U-O distances increase on reduction, from 1.80 to 1.90 Angstrom and from 2.43 to 2.50 Angstrom, respectively. The basic geometry of the U complex is apparently unchanged by reduction so any conformational change which does occur must be subtle.