A Zn-22 % Al eutectoid alloy was processed by equal-channel angular pressing (ECAP) to reduce the grain size to similar to 0.8 mu m. Tensile testing at 473 K showed superplastic characteristics with a maximum elongation of similar to 2230 % at a strain rate of 1.0 x 10(-2) s(-1). The significance of grain boundary sliding (GBS) was evaluated by measuring sliding offsets at adjacent grains from the displacements of surface marker lines in samples pulled to elongations of 30 % at a series of different strain rates. The highest sliding contribution was recorded under testing conditions corresponding to the maximum superplastic ductility. There were relatively large offsets at the Zn-Zn and Zn-Al interfaces, but smaller offsets at the Al-Al interfaces. Analysis shows the results are affected by the presence of agglomerates of similar grains which are present after ECAP processing and specifically by the increased fraction of Al-Al boundaries. The experimental results are in excellent agreement with the predictions of a deformation mechanism map depicting the flow behavior in the Zn-22 % Al alloy, and the results confirm the importance of GBS as the dominant mechanism of flow in superplasticity after processing by ECAP.