Abstract The superplastic deformation of fine-grained yttria-stabilized zirconia polycrystals (YSZPs) has been revised through a detailed analysis. The results indicate that high purity YSZPs (residual impurity content below 0.1 wt%) display a transition in the apparent stress exponent n ap from 2 (region II) to higher values (region I) with decreasing stress; a further change towards 1 (region 0) is found at very low stresses. A continuous variation in the apparent activation energy and grain size exponent with stress, grain size and temperature is found in region I. By contrast, low-purity YSZPs display only region II over the entire stress range. The threshold stress approach used to explain the conventional and high-strain-rate superplasticity of metallic alloys accounts for the mechanical characteristics of high-purity YSZP ceramics. The constitutive equation for superplastic flow is indeed identical with that found for metals when lattice diffusion is rate controlling. Yttrium segregation at grain boundaries is compatible with a threshold stress for grain-boundary sliding.