Low-temperature (c. 153 K) single-crystal X-ray structure determinations, carried out on trivalent rare earth iodides crystallized from aqueous solution at room temperature, have defined two series of hydrates, LnI(3 .)nH(2)O. For Ln = La-Ho, a nonahydrate phase (n = 9) is defined, orthorhombic Pmmn, a similar to 11.5, b similar to 8.0, Angstrom similar to 8.8 Angstrom, Z = 2, the second phase (n = 10), monoclinic P2(1)/c, Z = 4 being defined for Ln = Er-Lu, a similar to 8.2, b similar to 12.8, c similar to 17.1 Angstrom, beta similar to 103.7 degrees. Neither of these phases is isomorphous with any of those pertinent to the previously studied chloride or bromide (hydrated) arrays, nor, unlike those, does the halide (iodide) in any case enter the coordination sphere of the lanthanoid. The n = 9 phase takes the form [Ln(OH2)(9)]I-3, the nine-coordinate lanthanoid environment stereochemistry being tricapped trigonal-prismatic, while the n = 10 phase is [Ln(OH2)(8)]I(3 .)2H(2)O, the eight-coordinate lanthanoid environment being square-antiprismatic.