Sintered reaction-bonded silicon nitride (SRBSN) foams with very high porosity levels of up to 85 vol% and pore interconnectivity have been fabricated in this study. SRBSN foams were fabricated based on gel-casting of Si-PMMA suspensions followed by nitriding and sintering. The addition of MgO and Y2O3-Al2O3 as sintering additives resulted in significant changes in the foam microstructure. Small amounts of MgO completely prevented the whisker-forming reactions during nitridation while the addition of Y2O3-Al2O3 could not prevent whisker formation during nitridation. It was observed that regardless of the sintering aid system, SRBSN foams were whisker-free and revealed high pore interconnectivity after sintering. The SRBSN foam struts were only composed of beta-Si3N4 grains embedded in an intergranular amorphous phase.