We investigated the effects of a chemically-vapor-deposited mullite coating (similar to100 nm) on the oxidation resistance of sintered Si3N4 in air and steam environments. The coating was sacrificially incorporated into the thermally grown oxide (TGO) on Si3N4 during isothermal oxidation in air at 1400degreesC, leading to significantly reduced TGO growth as well as markedly improved TGO morphology. This improvement can be attributed to the refractory and viscous nature of the SiO2Al2O3 system, compared with SiO2, when under the influence of alkali and/or alkaline-earth fluxing elements. However, the mullite coating had little effect on the stability of the ceramic in the steam environment at 1200degreesC, due likely to high activity of SiO2 in mullite.