A correlation of densification behaviour and microstructural development of ZrO2-fluxed sintered reaction-bonded Si3N4 (SRBSN) is reported in the light of dilatometry and both high-resolution electron microscopy (HREM) and analytical electron microscopy (AEM). Two distinct dilatometer maxima were observed using a modified dilatometer with improved sensitivity. The relatively small first dilatometer maximum, at approximately 1 750-degrees-C, is due to the formation of a highly viscous silica-rich liquid phase at the initial stage of sintering. The second maximum, showing a more pronounced densification rate, is related to a radical change in secondary-phase chemistry at approximately 1900-degrees-C. In the SRBSN system, ZrO2 acts as an effective sintering aid because of the increased sintering temperatures and high N2 overpressure during the second dilatometer event, which promotes active participation of ZrO2 in the liquid-phase formation process. Sintering cycles were interrupted at the temperatures corresponding to the two dilatometer maxima. From these specimens, thin TEM foils were prepared, which correspond to the microstructures of the two densification events. Conventional TEM and HREM observations revealed significant microstructural differences, which could be related to the densification behaviour of the SRBSN-ZrO2 system. The good high-temperature performance of the densified SRBSN material is due to (i) the formation of in situ large-grown beta-Si3N4 grains, (ii) a completely crystallized secondary phase (m-ZrO2), and (iii) very thin amorphous phase- and grain-boundary films of approximately 1.0 and 0.5 nm, respectively.