Colorless crystals of the new hydroxometalates Ba[RE(OH)₅] with the rare-earth elements RE=Tb, Dy, Ho were synthesized under ultra-alkaline conditions in a KOH hydroflux at 200 °C. Single-crystal X-ray diffraction revealed that the three compounds crystallize isostructural in the monoclinic space group P2₁/n (no. 14). In the crystal structure, the rare-earth cations are coordinated by the oxygen atoms of seven hydroxide anions, which define a distorted pentagonal bipyramid. These polyhedra share edges of their basal ring forming infinite chains that run parallel to the [010] direction. Hydrogen bonds connect the chains into layers parallel to the (101) plane. The Ba²⁺ cations are located between these layers and surrounded by nine oxygen atoms. Ba[Dy(OH)₅] is paramagnetic and shows no luminescence under UV light. When heated in synthetic air or argon, water is released in well-defined steps. Ba[Dy(OH)₅] decomposes via DyOOH to Dy₂O₃, which then reacts with the remaining Ba(OH)₂ to form BaDy₂O₄. Thus, the hydroxometalates can be used as carbon-free precursors for oxides.