Superplastic deformation, grain boundary structure and cavitation behaviour during high temperature tensile testing in fine-grained 3 and 8 mol.% yttria-stabilized zirconia have been compared. The 8 mol.% yttria stabilized zirconia (8Y-CSZ) had a single cubic phase, while the 3 mol.% yttria-stabilized zirconia (3Y-TZP) was predominantly tetragonal. Extensive tensile ductility was obtained in 3Y-TZP, reaching similar to 283% at 1673 K and 1 x 10(-4) s(-1), whereas under the same test conditions, tensile elongation in 8Y-CSZ was limited to similar to 20%. These differences in elongation were related to differences in segregation of the yttria cation at grain boundaries. Transmission electron microscopy (TEM) observations with energy dispersive spectrometer (EDS) analyses revealed strong segregation of the yttria on grain boundaries in the 3Y-TZP; this segregation suppressed grain growth and lowered grain boundary mobilities. There was no evidence of preferential segregation of the yttria at grain boundaries in the 8Y-CSZ. Internal cavities developed in both materials during superplastic deformation. The results demonstrated that the extent of cavitation in 8Y-CSZ was much higher than in 3Y-TZP. In 8Y-CSZ, most of the cavities propagated in a direction perpendicular to the tensile axis as in most ceramics (with increasing strain these grew and interlinked to form cracks leading to failure at relatively low strains), whereas, in 3Y-TZP cavities were elongated parallel to the tensile axis as in metals. This difference arose from severe grain growth in 8Y-CSZ. (C) 2001 Elsevier Science B.V. All rights reserved.