The Hugoniots of the polymers polyethylene, polypropylene and polystyrene have been measured in terms of stress, shock velocity and particle velocity. All three materials have been shown to have a linear shock velocity–particle velocity relationship. This has been used to determine the hydrodynamic response, and compared with the measured shock stress. The results show that agreement between the two in all three materials is good, but at higher stresses, the hydrodynamic pressure is significantly lower. It has been suggested that this is due to an increasing shear strength with increasing impact stress. We have also observed that the difference between these two parameters increases with an increase in the size of the side group from the main polymer chain. We believe that this is due to an increasing resistance to compression caused by these side groups.