Thermodynamic calculations in the system Si--Al--O--N were used to assess the capabilities of SiO 2 and Al 2 O 3 mixtures to synthesise β -sialon powders by reduction-nitridation. The Thermo-Calc data-bank, a sophisticated computer software together with assessed thermodynamic data, has been applied to calculate not just traditional phase diagrams but also various sections, isopleths, activity plots and property diagrams such as phase fraction diagrams. By controlling the temperature, the partial pressures of oxygen and nitrogen and the Si:Al ratio in the precursor mixtures the experimental work can be reduced to a minimum, based on the calculations. β -Sialon was found to be stable only within a small region of oxygen and nitrogen partial pressures.