The high-temperature reaction of pure metals with silicon diimide Si(NH) ₂ in a specially developed radiofrequency furnace has been successfully applied to the synthesis of novel highly condensed nitridosilicates. With SrCO ₃ as an additional starting material this procedure has now been extended to the synthesis of oxonitridosilicates and oxonitridoaluminosilicates (sialons). Two novel sialons SrSiAl ₂ O ₃ N ₂ [space group P2 ₁ 2 ₁ 2 ₁ (no. 19), a=491.89(6), b=789.73(7), c=1134.94(18) pm, Z=4] and SrErSiAl ₃ O ₃ N ₄ [space group P6 ₃ mc (no. 186), a=606.53(3), c=985.90(8) pm, Z=2] have been obtained as coarsely crystalline materials. According to single crystal X-ray diffraction both compounds adopt structure types which are unprecedented for sialons. They derive from the nitridosilicates LnSi ₃ N ₅ (Ln=La, Ce, Pr, Nd) and MYbSi ₄ N ₇ (M=Sr, Ba, Eu), respectively, by partial substitution of Si by Al and N by O. For SrSiAl ₂ O ₃ N ₂ and SrErSiAl ₃ O ₃ N ₄ an unambiguous crystallographic differentiation between the tetrahedral centres (Al and Si) as well as the bridging atoms of the framework structures (N and O) seems reasonable.