²⁷Al high-resolution magic-angle spinning (MAS) nuclear magnetic resonance (NMR) experiments revealed that “MgAlON” spinel solid solutions have three different aluminium local structural environments. Two of them are characteristic of the MgAl₂O₄ spinel reference structure and involve of aluminium atoms in four- and six-fold oxygen co-ordinations. The presence of the third site is correlated firstly with oxygen/nitrogen substitution on the anionic lattice and secondly with the presence of magnesium in the spinel structure. This specific environment corresponds to a tetragonal site where the aluminium is surrounded by nitrogen atoms exclusively. Consequently, a model of solid solution is proposed for aluminium magnesium oxynitride and the chemical formulation of “MgAlON” samples is deduced.