An instrumentation system is described which provides on-line computation and direct readout of C and K. The system relies on the measurement of force and displacement or velocity across the sample and is independent of the type of test machine. Errors introduced by unwanted phase shift in any instrumentation system are discussed. This is overcome in this measurement system by phase compensation. Calibration of the system is described in which an overall system accuracy can be determined independent of test machine or rubber sample. This system has been specifically designed for measurement of dynamic properties of rubber and provides the operator with direct readout of C and K scaled to engineering units, thereby reducing potential operator error.