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There is an incentive to obtain data on the mechanical behaviour of superplastic materials to model superplastic forming processes, in particular to help reduce forming times. The usual method involves tensile tests at elevated temperatures without extensometry. However, inhomogeneous deformation has a large effect on the apparent stress-strain behaviour and elongation to failure. Strain distributions have been measured in tensile specimens of superplastic Al-4?5Mg alloy with different gauge lengths, and on a non-superplastic Al-1Mn alloy, all tested at 530uC. There was considerable drawing-in of material from the specimen ends to the gauge in the superplastic alloy, but not in the Al-Mn material. The draw-in meant that the apparent elongation of the Al-4?5Mg alloy almost doubled when the gauge length to width ratio was reduced from 4 to 1 and the apparent stress-strain behaviour was also significantly affected by the gauge geometry.