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The reactions of RE(IO[sub 3])[sub 3] [RE = Nd, Sm, Eu] with I[sub 2]O[sub 5] and MoO[sub 3] in a 1:2:2 molar ratio at 200 °C in aqueous media provide access to RE(MoO[sub 2])(IO[sub 3])[sub 4](OH)[RE = Nd (1), Sm (2), Eu (3)] as pure phases as determined from powder X-ray diffraction data. Single crystal X-ray diffraction experiments demonstrate that these compounds are isostructural and crystallize in the chiral and polar space group P2[sub 1]. The structures are composed of three-dimensional networks formed from eight-coordinate, square antiprismatic RE[sup 3+] cations and MoO[sub 2](OH)[sup +] moieties that are bound by bridging iodate anions. The Mo(Vl) centers are present in distorted octahedrat environments composed of two cis-oxo atoms, a hydroxo group, and three bridging iodate anions arranged in a fac geometry. There are four crystallographically unique iodate anions in the structures of 1-3, one of these is actually present in the form of a IO[sub 3+1] polyhedron where a short interaction of 2.285(4) Å is formed between the iodate anion and the hydroxo group bound to the Mo(Vl) center. This interaction results in significant distortions of the iodate anion similar to those found in tellurites with TeO[sub 3+1] units. Two of the four iodate anions are aligned along the polar b-axis, imparting the required polarity to these compounds. Second-harmonic generation (SHG) measurements on sieved powders of 1 show a response of 350× α-quartz. Crystallographic data: 1, monoclinic, space group P2[sub 1], a = 6.9383(5) Å, b = 14.0279(9) Å, c = 7.0397(5) Å, β = 114.890(1)°, Z = 2; 2, monoclinic, space group P2[sub 1], a = 6.9243(6) Å, b = 13.963(1) Å, c = 7.0229(6) Å, β = 114.681(1)°, Z = 2; 3, monoclinic, space group P2[sub 1], a = 6.9169(6) Å, b = 13.943(1) Å, c = 7.0170(6) Å, β = 114.542(1)°, Z = 2. [ABSTRACT FROM AUTHOR]