The kinetic performance of metallocene type catalysts as well as their instantaneous activity is determined on line by two independent methods in the semi-batch polymerization of ethylene via metallocenes. The first-principle basis of both methods is described and guidelines for their implementation at a laboratory scale reactor are offered. Polymerization tests were conducted with two heterogenized metallocene catalysts showing that the direct method (based on ethylene flow measurement) and the calorimetric method (based on energy balances and developed here) report equivalent high quality information. This last method can be readily used by the chemical practitioner as the notions and tools required for its implantation are easily grasped; it also has the advantage of requiring a low cost instrumentation (only thermocouples), whereas the direct method needs a relatively more sophisticated equipment (mass flow meter). © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2014, 131, 40035.