C and SiO2of different types were mixed and heat‐treated (1410° to 1550°C), according to a standardized temperature program, in flowing N2at different pressures. It was found that a starting material combination of C (115 m2/g) and SiO2(50 m2/g) yielded pure Si3N4after about 2 h at 1550°C and 1.3 MPa. By adjustment of the pressure in the range 2 to 6 MPa, irreversible evaporation of SiO(g) as well as formation of nonequilibrium phases was suppressed, and the amounts of residual C and O in the sample were controlled. Even a small amount of CO in the N2was observed to retard the nitridation. Possible explanations are discussed, based on thermodynamic calculations on the Si‐O‐N