The high temperature alkali corrosion kinetics of SiC and Si3N4 were investigated from 950 degrees C to 1100 degrees C in dry air containing 0.98 vol.% alkali. For both materials linear reaction rates were observed at all temperatures indicating that the alkali corrosion of SiC and Si3N4 is surface reaction controlled with activation energies of 104 and 199 kJ/mol, respectively. The average composition of the sodium silicate reaction layer was (Na(2)0).(SiO2)(2.22) for Si3N4 and (Na(2)0).(SiO2)(2.55) for SiC as determined by atomic absorption analysis. The overall corrosion reactions are complex involving absorption, decomposition, dissolution, diffusion and surface reaction processes, but the rate-controlling step in the corrosion process for both materials appears to be the interfacial oxidation of SiC or Si3N4 to SiO2. A comparison of the reaction kinetics and mechanism between the oxidation of each material with and without alkalis present is presented.