The chemistry of glass-ceramic interface was studied in two different diffusion couples, YSiAlO/Si3N4 and SiO2/Si3N4. Experiments were performed at 1500degreesC for 2 hours in argon atmosphere. The couples were analyzed through SEM and EDS to draw the concentration profiles across the interface. It was observed that aluminosilicate glass infiltrates within the ceramic secondary phase and detaches the silicon nitride grains. The free grains move toward the glass surface either. by solution-reprecipitation or floating. The glass also dissolves silicon nitride grains and the nitrogen is absorbed, forming a broad zone of YSiAlON glass. In the SiO2/Si3N4 couple there was extensive. cation diffusion from the silicon nitride grain boundary to the silica glass, forming an aluminosilicate intermediary phase. It was observed crystallization of some yttrium-rich phase at the interface.