The paper presents the results of the analysis of the state diagram of compounds in the system А12O3−SiO2. It has been found that the presence and the concentration of oxygen have a very important effect on formation of compounds with a crystalline structure in different syngony based on SIALON. Oxygen contributes to transition of the metastable AlXSi3−XN4compound into stable one. The parameter of structural “friability” of compounds has been used in the analysis of the thermodynamic stability of compounds in the Al-Si-N-O system. It has been found that the SiAl4O2N4 compound with the 12H-SIALON structure (Pearson symbol hP32) has the greatest thermodynamic stability among the compounds under study in this system.