Amorphous grain boundary phases in 3-mol%-yttriastabilized zirconia ceramics (3Y-TZP) were studied to determine the influence of intergranular amorphous silicate phases on tensile superplasticity at temperatures of 1300-1500 degreesC. Controlled additions (1 wt%) of compositionally distinct barium silicate and borosilicate phases were used. The initial grain sizes of the pure, barium silicate added, and borosilicate-added samples were 0.45, 0.55, and 0.55 mum, respectively. Systems with added barium silicate and borosilicate glass both exhibited a 60% reduction in flow stress as compared with pure 3Y-TZP, with the lower-viscosity barium silicate system exhibiting a slightly greater reduction in flow stress. The higher-viscosity borosilicate glass/3Y-TZP materials exhibited the greatest elongation to failure, while the barium silicate/3Y-TZP materials had the least elongation. Yttrium was found to segregate to grain boundaries in the pure and borosilicate-containing samples, and both yttrium and barium were found to segregate to grain boundaries in the barium silicate containing samples. No silicon was observed along two-grain boundaries in any of the samples, even those containing pockets of glass, The difference in deformation behavior may be due to a combination of viscosity of the glass addition, grain boundary segregation, and grain boundary bond character.