Abstract: A number of new solid solutions Ca_{3 − 1.5x}Er_x□_0.5xB₂SiO₈ (x = 0–0.4) based on borosilicate Ca₃B₂SiO₈ are synthesized: homogeneous solid solutions with х = 0–0.125 are structurally similar to the low-temperature monoclinic polymorphic modification of β-Сa₃B₂SiO₈; and at 0.16 < х < 0.25, to the high-temperature orthorhombic polymorph α-Сa₃B₂SiO₈. High-temperature X-ray powder diffraction and high-temperature Raman spectroscopy (20–1000°C) are used to study the monoclinic-orthorhombic transition of solid solutions through the two-phase region (0.125 < х <0.16), thermal expansion of monoclinic and orthorhombic solid solutions (x = 0.1, 0.15), and their decomposition with the formation of silicate Ca₂Er₈(SiO₄)₆O₂ above 990°C. According to the data of the optical spectroscopy, it is found that the intensity of the photoluminescence of erbium ions is significantly lower than the intrinsic luminescence of the borosilicate matrix in the entire studied range of concentrations of active ions.