The reactions of (NpO2)-Np-237 with excess iodate under acidic hydrothermal conditions result in the isolation of the neptunium(IV), neptunium(V), and neptunium(VI) iodates, Np(IO3)(4), Np(IO3)(4)center dot nH(2)O center dot nHIO(3), NpO2(IO3), NpO2(IO3)(2)(H2O), and NpO2(IO3)(2)center dot H2O, depending on both the pH and the amount of water present in the reactions. Reactions with less water and lower pH favor reduced products. Although the initial redox processes involved in the reactions between (NpO2)-Np-237 or (PuO2)-Pu-242 and iodate are similar, the low solubility of Pu(IO3)(4) dominates product formation in plutonium iodate reactions to a much greater extent than does Np(IO3)(4) in the neptunium iodate system. UO2 reacts with iodate under these conditions to yield uranium(VI) iodates solely. The isotypic structures of the actinide(IV) iodates, An(IO3)(4) (An = Np, Pu), are reported and consist of one-dimensional chains of dodecahedral An(IV) cations bridged by iodate anions. The structure of Np(IO3)(4)center dot nH(2)O center dot nHIO(3) is constructed from NpO9 tricapped-trigonal prisms that are bridged by iodate into a polar three-dimensional framework structure. Second-harmonic-generation measurements on a polycrystalline sample of the Th analogue of Np(IO3)(4)center dot nH(2)O center dot nHIO(3) reveal a response of approximately 12x that of alpha-SiO2. Single-crystal magnetic susceptibility measurements of Np(IO3)(4) show magnetically isolated Np(IV) ions.