Six oxide and oxynitride glasses were synthesized in the Y–Mg–Si–Al–O–N, Nd–Mg–Si–Al–O–N and La–Mg–Si–Al–O–N systems. As already known, nitrogen introduction increases the T _g, packing factor and mechanical properties of the glasses. Cationic substitution also has an influence on the glasses’ behavior, particularly in terms of sensitivity to indentation load/size effect (ISE). The structure of the yttrium-containing glasses was investigated by mean of ²⁷Al and ²⁹Si MAS-NMR. Al is found to occur for 2/3 as a network former and for 1/3 as a modifier. The oxide glass mainly contains Q² and Q³ silicate units and SiO₃N and SiO₂N₂ units are created when nitrogen is introduced into the glass network. The average number of rigid bonds per network former was calculated from the glasses’ composition. A discrepancy between and the Raman spectra of the glasses suggests that parts of the magnesium behaves as a former in the network. seems to be a key parameter governing hardness and sensitivity to ISE and can be linked to normal/abnormal behavior of glasses regarding indentation.