Four new Dy and Nd hydrated octamolybdate materials have been prepared: [Dy-2(H2O)(12)](Mo8O27)center dot 8H(2)O, [Dy-2(H2O)(6)](Mo8O27), [Nd-2(H2O)(12)](Mo8O27)center dot 6H(2)O, and [Nd-2(H2O)(6)]Mo8O27 center dot 3H(2)O. They adopt one known and three new structure types, which we have determined ab initio from powder X-ray diffraction data. The four compounds contain the same basic structural building block, in the form of RE2Mo8O27 (RE = Dy, Nd) chains, which are arranged differently for the two rare earths in the fully hydrated precursor materials. [Dy-2(H2O)(12)](Mo8O27)center dot 8H(2)O comprises isolated Dy2Mo8O27 chains assembled in a zigzag manner along the a axis with interspersed crystallized water molecules, which differs from the parallel arrangements of isolated Nd2Mo2O27 chains in [Nd-2(H2O)(12)](Mo8O27)center dot 6H(2)O. Partial dehydration of the two precursors leads to new phases [Dy-2(H2O)(6)](Mo8O27) and [Nd-2(H2O)(6)]Mo8O27 center dot 3H(2)O, respectively, prior to complete dehydration, which leads to initially amorphous and eventually crystalline rare-earth molybdenum mixed metal oxides. Both initial transformations occur topotactically. Partial dehydration of the Dy phase condenses the assembly of the Dy2Mo8O27 chains principally along the a axis, leading to a 3-dimensional (3D) framework, with each Dy bridging two (Mo8O27)6 sheets in the product. The partial dehydration of the Nd precursor condenses the assembly of Nd2Mo8O27 chains along the [110] axis, leading to a 3D framework where each Nd bridges three (Mo8O27)(6-) units. Both new dysprosium octamolybdates adopt noncentrosymmetric polar structures in space group F2(1) and are second harmonic generation (SHG) active.