The oxonitridosilicate oxides RE26Ba6[Si22O19N36]O-16:Eu2+ (RE = Y, Tb) were synthesized by high-temperature reaction in a radiofrequency furnace starting from REF3, RE2O3 (RE = Y, Tb), BaH2, Si(NH)(2), and EuF3. The structure elucidation is based on single-crystal X-ray data. The isotypic materials crystallize in the monoclinic space group Pm (no. 6) [Z = 3, a = 16.4285(8), b = 20.8423(9), c = 16.9257(8) angstrom, beta = 119.006(3)degrees for RE = Y and a = 16.5465(7), b = 20.9328(9), c = 17.0038(7) angstrom, beta = 119,103(2)degrees for RE = Tb]. The unique silicate layers are made up from Q(1)-, Q(2)-, and Q(3)-type Si(O/N)(4)- as well as Q(4)-type SiN4-tetrahedra, forming three slightly differing types of cages. The corresponding 3-fold superstructure as well as pronounced hexagonal pseudosymmetry complicated the structure elucidation. Rietveld refinement on powder X-ray diffraction data, energy-dispersive X-ray spectroscopy and infrared spectroscopy support the findings from single-crystal X-ray data. When excited with UV to blue light, Y26Ba6[Si22O19N36]O-16:Eu2+ shows broad orange-red luminescence (lambda(em) = 628 nm, fwhm approximate to 125 nm/3130 cm(-1)). An optical band gap of 4.2 eV was determined for the doped compound by means of UV/vis spectroscopy.