The novel oxonitridoaluminosilicates (sialons) Sr₃Ln₁₀Si₁₈Al₁₂O₁₈N₃₆ (Ln = Ce, Pr, Nd) were obtained by the reaction of the respective lanthanide metals with Si(NH)₂, SrCO₃, and AlN using a radiofrequency furnace at temperatures between 1550–1650 °C. The crystal structures of the isotypic sialons were determined by single-crystal X-ray investigations (Sr₃Ce₁₀Si₁₈Al₁₂O₁₈N₃₆: I3m, Z = 2, a = 1338.2(2) pm, R1 = 0.0333; Sr₃Pr₁₀Si₁₈Al₁₂O₁₈N₃₆: a = 1334.54(6) pm, R1 = 0.0296; Sr₃Nd₁₀Si₁₈Al₁₂O₁₈N₃₆: a = 1332.85(6) pm, R1 = 0.0271) and in the case of Sr₃Pr₁₀Si₁₈Al₁₂O₁₈N₃₆ with powder neutron diffraction as well. The three-dimensional sialon network is built up by SiON₃, SiN₄, and AlON₃ tetrahedra. Besides the bridging O and N atoms of the sialon network there are isolated O²⁻ which are tetrahedrally coordinated by Sr and Ln. The crystallographic differentiation of Si/Al and O/N seemed to be possible by a careful evaluation of the single-crystal X-ray diffraction data combined with lattice energy calculations using the MAPLE concept (Madelung Part of Lattice Energy). In the case of Sr₃Pr₁₀Si₁₈Al₁₂O₁₈N₃₆ the differentiation of O and N and the proposed ordering was completely confirmed by powder neutron diffraction.