A test sample of raw or compounded rubber is squeezed from a loading chamber (7) through selectively-aligned passages (8,12) to fill an annular space (1) between a coaxial stator (5) and rotor (4). Measurements of viscous torque are provided by a sensor (15), and a load cell (14) provides measurements of the force exerted axially by the sample. The torque and axial-force measurements are plotted against time (16 FIG. 4, 17 FIG. 5) as representative respectively of the viscosity V and elasticity of recovery E of the sample, and are compared for correspondence (within limits 19, 20) with earlier-plotted characteristics of an acceptable rubber for processing. Temperature of the sample is regulated by a heater (18) and rotation of the rotor (4) is at a speed and for a duration corresponding to that of the next processing stage.