In the present work, the influence of the SiO2 addition on the decomposition reaction is studied. with this aim a wide composition range is covered, going from a content of silica barely superior to the commercial powder until reaching the necessary amount to get the stoichiometric ratio for the silicon oxynitride formation. The results obtained for sintering without pressure of Si3N4 with SiO2 additions are analyzed in function of the weight loss and these show that it is not the Si3N4 decomposition the only reaction responsible for the weight loss. The weight loss rate increase with SiO2 content. Additionally, sintering cycles without pressure in nitrogen atmosphere using powder beds (50 wt.% Si3N4 + 50 wt.% BN) were made. The use of nitrogen atmosphere and powder beds tends to avoid the Si3N4 decomposition.