New solid solutions Ca₃B_{2 + x}Si_1–xO_8–x/2 based on monoclinic borosilicate β-Ca₃B₂SiO₈ are prepared using crystallization from a melt. The homogeneity range of solid solutions (0 < х ≤ 0.86) is determined and the thermal behavior of solid solution Ca₃B_2.4Si_0.6O_7.8 is studied using high-temperature X-ray powder diffraction: a reversible β ↔ α polymorphic transition is discovered in the solid solution (х = 0.4) at 550°C, decomposition begins in the solid state above 940°C with the formation of CaSiO₃. The thermal expansion of both modifications is anisotropic: 20–540°C (α₁₁ = 17, α₂₂ = 16, α₃₃ = 0, α_V = 32 × 10⁻⁶°C⁻¹) and 560–900°C (α₁₁ = 15, α₂₂ = 24, α₃₃ = 3, and α_V = 42 × 10⁻⁶°C⁻¹).