Abstract: A rheometer of the type which includes a chamber (26) in which, in use, a sample of rubber or of a synthetic elastomer which is to be tested is held under pressure in contact with a rotor (27), and means for measuring torque applied to the rotor (27) as it is rotated in contact with the sample, is provided with a driving mechanism (35) for rotating the rotor (27) driven by two pulse-operated electric stepping motors (40, 41) operating through pinions (44, 49) and a differential gear (46). The use of two stepping motors (40, 41) and the differential gear (46) enables the rheometer to test the sample in various ways. Firstly the rotor (27) may be rotated in one direction to strain the sample beyond its maximum recovery and then be rotated in a reverse direction step by step until there is zero torque on the rotor and the total reverse rotation then indicates the total angular recovery of the sample.