Twelve different silicon nitride-disilicate ceramics have been fabricated by hot-pressing Si3N4 with the oxides of Y, Yb, Ho, Dy, Er, Sm, Ce, Lu, La, Pr, Gd, and Sc that are used as sintering additives. The high temperature strength and oxidation behaviour of the hot-pressed ceramics were investigated and correlated with the cationic radii of the oxide additives. The flexural strength at 1200 degrees C increased, from 666 MPa for Si3N4-La2Si2O7 to 965 M Pa for Si3N4-Sc2Si2O7 which is correlated with a decreasing cationic radius of the oxide additive. The weight gain during oxidation at 1400 degrees C for 192 h in air decreased, from 0.8732 mg cm(-2) for a Si3N4-Sm2Si2O7 ceramic to 0.1089 mg cm(-2) for a Si3N4-Sc2Si2O7 ceramic, which is a function of the decreasing cationic radius of the oxide additive.