Whereas the moduli of elastic solids and the viscosities of simple molecular liquids are well-defined, such that many can be used as standards, the same is not true of viscoelastic materials. The properties of the latter are by definition both frequency and temperature dependent. In this paper some candidate materials and the results obtained over a wide (−100 to +100 °C) temperature range and a fairly wide measurement frequency range (0.01–100 Hz) are considered. In order to extend this frequency range, particularly into the kHz range, the time–temperature superposition ideas of Williams, Landel, and Ferry are used. This necessitates the material conforming to this treatment by virtue of possessing a temperature-independent relaxation spectrum, to a good approximation. In order to have relatively high damping at ambient, the polymer has to have a Tg around 0 °C, and, to be thermorheologically simple, it has to be predominantly single phase. The best candidate materials have been found to be random copolymers of butadiene and acrylonitrile.