A detailed study of the macroscopic and microscopic behavior upon sintering of sodium niobate was carried out. A dynamic sintering curve, recorded at a constant heating rate of 5 degrees C/min up to 1412 degrees C, supported by the microstructural analysis of the samples, quenched at selected temperatures, revealed a multiple-stage densification. This was connected with the activation of different material-transport mechanisms: evaporation/condensation and surface diffusion up to 950 degrees C, and other diffusion mechanisms, which contribute to both shrinkage and grain growth at higher temperatures. The second series of quenching experiments was used to track the microstructure development during the isothermal heating stage of the sintering process at 1350 degrees C. The change from a fine-grained microstructure, with an average grain size of 2.53 +/- 1.1 mu m, to a coarse-grained microstructure, with an average grain size of 35.6 +/- 32 mu m occurred after isothermal sintering for 30 min. The first entrapped pores within the large grains of several 10 lm indicated the increased grain-boundary migration velocity already after 15 min of sintering. These conditions were identified as the onset point for abnormal grain growth in sodium niobate.