Norton NBD 200 silicon nitride ceramics were implanted with sodium to a dose of 7.0 x 10(15) cm(-2) at 72 keV (1 at % peak sodium content at 100 nm). The sodium-implanted samples were further implanted with aluminium to 7.3 x 10(-15) cm(-2) at 87 keV (1 at % peak aluminium content at 100 nm). The implanted and unimplanted samples were oxidized in 1 atm dry oxygen at 1100 and 1300 degrees C for 2-6 h. Profilometry and scanning electron microscopy measurements indicated that sodium implantation led to up to a two-fold increase in the oxidation rate of silicon nitride. The sodium effect was effectively neutralized when aluminium was co-implanted. The opposite effects of sodium and aluminium on the oxidation resistance of silicon nitride can be attributed to their different roles in modifying the structure and properties of the oxide formed.