Experimental data are given in this paper showing the variation of stiffness, hysteresis energy absorbed per cycle, and resilience, with frequency, amplitude of deformation, and temperature. The data have been taken from the results of a series of tests covering the following ranges: Frequency 1.33–1,200 cycles per min., amplitude 0–10 per cent of the free length of the test-piece, and temperature from 125 deg. C. to −60 deg. C. (257 deg. F. to −76 deg. F.). In the latter part of the paper an attempt has been made to develop expressions to agree with the dynamic behaviour of rubberlike materials.