Targets of 2024‐T351 aluminum were shocked to approximately 110 kbar and 340 kbar by flyer plates having velocities of 0.12 and 0.32 cm/&mgr;sec, respectively. Free‐surface velocities were determined as a function of target thickness by recording the time of flight across known distances of thin shims which were originally in intimate contact with the surfaces of the samples. The experimental data are believed to be more accurate than any obtained previously. In earlier work it appeared that the free‐surface velocity decreased in a stepwise manner as the target thickness was increased. The new data do not show a stepwise decrease, so the simple elastoplastic relations cannot be used to predict attenuation. Some improvement in the predictions was obtained by using a variable shear modulus. The relation between the shear modulus and the strain was obtained from the results of the attenuation experiments. Further improvement may be obtained by the inclusion of the Bauschinger effect in the calculations. Some data were obtained for annealed 1060 aluminum at 110 kbar. The response of 1060 aluminum appears to differ significantly from that of the hard aluminum.