The effects of different heat-treatment temperatures and gases on the crystallisation of Si₃N₄–Al₂O₃–SiO₂–Y₂O₃ glasses were investigated. Simultaneous thermal analyses and high temperature calorimetry were carried out to determine the glass transition temperatures, the crystallisation temperatures and enthalpies. In situ high temperature X-ray powder diffraction was used to determine the crystallisation behaviour of these glasses while X-ray diffraction studies were carried out to find the volume fraction of phases. The qualitative and quantitative phase analyses were conducted using scanning electron microscopy combined with energy dispersive analysis and electron microprobe. It was found that the gas type, the ageing temperature and the nitrogen content were greatly affecting the microstructure and the glass transition temperatures in Si₃N₄–Al₂O₃–SiO₂–Y₂O₃ glass–ceramics and produced different modifications of yttrium disilicate. Moreover, it was observed that the formation of D-phase (YSi₂AlO₄N₂) depends on the initial nitrogen contents in the samples.