Pore boundary tessellation and quantitative stereology were used to characterize microstructure evolution in ZrO2-3 mol%Y2O3 (3YSZ) that had been pressed to a green density of 46% and isothermally sintered at 1275 degrees C for 0.1-10 h. Scanning electron micrographs showed that, relative to classical sintering models, the sintered 3YSZ microstructure was spatially heterogeneous, and that this heterogeneity affected the way in which the microstructure evolved during sintering. Pore boundary tessellation cell maps revealed the presence of dense regions within the microstructure that grew by the preferential elimination of smaller pores and resulted in larger more widely spaced porosity at longer sintering times. In consequence, the average pore separation distance increased much faster than the average grain size. This would call into question the use of the grain size as a measure of microstructural scale for the prediction of densification kinetics for this material.