By means of the split Hopkinson pressure bar (SHPB) technique, the time domain dynamic mechanical properties of a polyisoprene elastomer were characterized over a range of temperatures. These properties include the dynamic stress- strain and compressional relaxation modulus characteristics of the elastomer. In the SHPB technique employed in the measurements, two identical long steel bars, which are known as the incident and transmitter pressure bars, were used as wave guides. Solid disc specimens of 10 mm diameter by 3 mm and 6 mm thickness of the polyisoprene rubber were sandwiched, in turn, between the bars. Strain pulses were generated in the incident pressure bar by the collinear impact of a hardened steel spherical ball, which was fired from a mechanical launcher, with the plane free end of the incident pressure bar via a small cylindrical anvil which was attached to the impacted end of the incident pressure bar. The strain pulses generated and propagated down the pressure bar were incident on, reflected from and transmitted through the polyisoprene specimen. These pulses were monitored by PZT sensors of dimensions 5 mm by 3 mm, which were bonded to the middle locations of the pressure bars, and were used to derive the dynamic properties of the specimens. It is shown that the stress and compressional relaxation modulus characteristics of this elastomer undergo larger variations and attain higher values at low temperatures than at high temperatures.