The phenomenon of superplasticity has been demonstrated in several zirconia-alumina composites. However, the rate controlling mechanism has not yet been unambiguously identified, due to the limited data available on these materials in comparison with 3 mol% yttria stabilized tetragonal zirconia (3YTZ). The limited data on a zirconia-20 wt% alumina (3Y20A) composite suggest that the mechanical characteristics are similar to those of 3YTZ. The present experimental study on 3Y20A reveals the occurrence of diffusion creep. The experimental results are examined critically in terms of dislocation activity and diffusion creep, and their relevance to superplastic deformation.