On the basis of thermodynamic calculations, Bi₈(SeO ₃)₉Br₆ was synthesized from BiOBr and SeO ₂ via an autotransport reaction over a temperature gradient of 670 to 660 K. Crystals of CsSm₂₁(SeO₃)₂₄Br ₁₆ were obtained from the reaction of an equimolar mixture of SmOBr and SeO₂ in a CsBr flux at 1070 K. The two selenite bromides crystallize in the orthorhombic space group B 2/b 2/a 2/b (Nr. 68) with pseudo-tetragonal lattices (Bi₈(SeO₃)₉Br ₆: a = b = 1588.4(5), c = 1830.5(5) pm, Z = 5.333; CsSm ₂₁(SeO₃)₂₄Br₁₆: a = b = 1579.7(2), c = 1796.3(2) pm, Z = 2). In the crystal structures, the heavy atoms form square nets. The O atoms of the pyramidal selenite groups connect them into slabs of composition [Bi^III_21.33(Se^IVO ₃)₂₄Br₁₆] or [CsSm^III ₂₁(Se^IVO₃)₂₄Br₁₆], which are stacked along [001]. In CsSm₂₁(SeO₃) ₂₄Br₁₆, electroneutrality is achieved through the substitution of one Sm^III cation per formula unit by a Cs ^I cation. In the case of Bi₈(SeO₃) ₉Br₆ = Bi^III_21.33(Se ^IVO₃)₂₄Br₁₆, the partial depopulation of Bi positions maintains charge balance. The intrinsic symmetry of the slabs, which corresponds to layer group P (4/n) 2/b 2/m, results in pseudo-merohedral twins with twin axis [110].