Two compositions of the Y2O3-MgO (YM) and Yb2O3-MgO (YbM) systems were chosen to study the effect of the sintering additive composition on the processing and thermal conductivity of sintered reaction-bonded silicon nitride (SRBSN). The nitridation, postdensification, microstructural evolution, and thermal conductivity of SRBSN were found to depend strongly on the sintering additive composition. The RBSN materials with YbM exhibited a poor sinterability, whereas those with YM exhibited an excellent sinterability. However, the SRBSN materials with YbM showed a higher thermal conductivity than those with YM. This was associated primarily with the isolated distribution and lower amount of secondary phase and the higher percentage of large grains in the former materials.