Al-Cu alloys containing 6, 11, 17, 24 and 33 wt% Cu, annealed for 0.5-100 h, were deformed by the differential strain-rate test technique over a strain-rate range of ≈4×10-6 to 3×10-2s-1 at temperatures ranging from 460-540°C. Superplastic behaviour, with strain-rate sensitivity, m≈0.5, and activation energy, Q=171.5 kJ mol-1, is shown by the Al-24Cu and Al-33Cu alloys at lower strain rates and higher temperatures. All the alloys show m≲0.20 at higher strain rates, but the average activation energy for deformation of the Al-6Cu, Al-11Cu, and Al-17Cu alloys is evaluated to be 480.7 kJ mol-1, in contrast to a lower value of 211 kJ mol-1 for the Al-24Cu and Al-33Cu alloys. Instead of grain size, the mean free path between θ particles is suggested to be a more appropriate microstructural parameter for the constitutive relationship for deformation of the Al-Cu alloys. © 1995 Chapman & Hall.