Polycrystalline CsBSi₅O₁₂ was prepared from a stoichiometric mixture by solid-state reaction above 1000 °C. The solid solutions Cs_1–xRb_xBSi₅O₁₂ were obtained at 1000 °C during a long heat treatment of polycrystalline Cs_1–xRb_xBSi₂O₆ boropollucites (x _Rb = 0, 0.05, 0.2, 0.4). A new borosilicate compound and its solid solutions were studied using X-ray powder diffraction (XRD), annealing, differential scanning calorimetry (DSC), and thermogravimetry (TG). For Cs,Rb-boropollucites the new phase formation is accompanied by significant mass losses detected by DSC and TG. The following mechanism of phase transformations is assumed: (Cs,Rb)BSi₂O₆ → (Cs,Rb)BSi₅O₁₂ + (Cs,Rb)BO₂↑. The zeolite phase forms as a result of the boropollucite decomposition over 1000 °C. Zeolite decomposes also on further heating and the SiO₂ reflections are observed in the XRD pattern only. Thus above 1000 °C both boropollucite and zeolite phases are unstable presumably due to the ability of the alkali cations to leave the structure. Using XRD the unit cell parameters of CsBSi₅O₁₂ have been determined in the orthorhombic crystal system: a = 16.242(4) Å, b = 13.360(4) Å, c = 4.874(1) Å. The compound is isostructural with the zeolite compound CsAlSi₅O₁₂. In the crystal structure of Cs_1–xRb_xBSi₅O₁₂ solid solutions the changes of cell parameters are insignificant under the substitution of Cs by Rb atoms that indicates a very limited substitution range.