Three series of Sr_3–1.5xEu_xB_2+ySi_1–yO_8–y/2 (xEu³⁺ = 0.01–0.51; y = 0, 0.28, 0.53) solid solutions were prepared by crystallization of the melt. The crystal structures of solid solutions with x = 0.2, y = 0; x = 0.06, y = 0.28; x = 0.23, y = 0.28; x = 0.46, y = 0.28; x = 0.06, y = 0.53 and x = 0.51, y = 0.53 were determined on the basis of single-crystal X-ray diffraction data (XRD) and refined in the orthorhombic space group Pnma to R_obs = 0.029, 0.054, 0.030, 0.029, 0.042 and 0.025, respectively. Heterovalent substitution scheme 3Sr²⁺ → 2Eu³⁺ + □ is assumed from the single-crystal XRD data. In the phases with xEu³⁺≥ 0.2 the substitution leads to a split of the Sr/Eu positions. According to the results of powder and single crystal study, the critical concentration of europium in the structure reaches about 17 at. %. The Eu-doped borosilicates are thermally stable upon heating in air up to at least 800 °C, but decompose at above 900 °C forming a mixture of borates and silicates of Sr and Eu. Ceteris paribus, fluorescence intensity of the borosilicates with y = 0 and 0.28 shows a similar high level of fluorescence while that of the series enriched in boron (y = 0.53) is reduced. The measured quantum yield of the promising red phosphors (xEu³⁺ ≥ 0.4) demonstrates high values from 44 up to 79% depending on Sr/B/Si ratio.