Bismuthates with composition M₃BiBr₆ (M=Rb, Cs) form series of polymorphs capable of reversible H₂O incorporation at room temperature. The host compounds were synthesized by annealing mixtures of MBr/BiBr₃, or by precipitation from acidic solution and removing water molecules from the host framework of the hydrates in a thermal dehydration reaction. While the compound oP-Rb₃BiBr₆ (space group Pbnm) is known from previous work, mP-Rb₃BiBr₆ crystallizes with a new type of structure (space group P2₁/c). For Cs₃BiBr₆, the oP-phase crystallizes isostructural to Cs₃NdCl₆ (space group Pbcm), the mC-phase isostructural to Cs₃BiCl₆ (space group C2/c) and the tP-phase isostructural to Tl₃BiCl₆ (space group P4₂/m). All crystal structures were refined from single crystal X-ray diffraction data. In both systems, the crystal structures of orthorhombic and the monoclinic phases are polytypes and differ by stacking sequence only. By intercalation of water molecules, all polytypes of M₃BiBr₆ transform to semi-hydrates M₃BiBr₆ ⋅ (Formula presented.) H₂O. Both, the incorporation of water and the reverse process proceed without the host structure being changed. The inclusion of crystal water was also confirmed by IR-spectroscopy, thermal gravimetric analysis and mass spectrometry. The modification tP-Cs₃BiBr₆ features a crystal structure with higher density and does not accommodate water.