The structure and properties of an oxonitridosilicate ('sion') and an oxonitridoaluminosilicate ('sialon') have been studied using density functional theory (DFT). The bulk moduli and elastic stiffness coefficients are predicted. We show that the very high hardness of SrSiAl2O3N2 cannot be explained by its elastic properties. The enthalpy difference between two structural models for Ce-4[Si4O4N6]O has been determined to be 8 kJ/mol, implying that at ambient temperature there could be a partial disorder. Interatomic bonding has been investigated by a Mulliken population analysis and the experimental finding of an anomalously long Si-O bond in SrSiAl2O3N2 is discussed.