The oxonitridocarbidosilicates RE4Ba2[Si12O2N16C3]:Eu2+ (RE = Lu, Y) were synthesized by carbothermal reactions starting from RE2O3, graphite, Ba2Si5N8, Si(NH)(2), and Eu2O3. The crystal structure of Lu4Ba2[Si12O2N16C3]:Eu2+ was elucidated on a submicron-sized single crystal by a combination of transmission electron microscopy and microfocused synchrotron radiation. The compound crystallizes in trigonal space group P3 (no. 143) with a = 16.297(4) angstrom, c = 6.001(2) angstrom, and Z = 3 (R1 = 0.0332, wR2 = 0.0834, GoF = 1.034). According to Rietveld refinements on powder X-ray diffraction data, Y4Ba2[Si12O2Ni6C3]:Eu2+ is isotypic with a = 16.41190(6) angstrom and c = 6.03909(3) angstrom. The crystal structures are built up of vertex-sharing SiC(O/N)(3) tetrahedra forming star-shaped units [C-[4](Si(O/N)(3))(4)] with carbon atoms in fourfold bridging positions. Energy-dispersive X-ray spectroscopy and to the sum formula, lattice energy, and charge distribution calculations support the assignment of O/N/C atoms. When excited with UV to blue light, Eu2+-doped samples show green luminescence for RE = Lu (lambda(em) approximate to 538 nm, full width at half-maximum (fwhm) approximate to 3600 cm(-1)) and yellow emission in the case of RE = Y (lambda(em) approximate to 556 nm, fwhm 4085 cm(-1)).