A novel lab-scale stirred bed gas phase reactor system has been designed and constructed for investigating the kinetic behaviour of olefin copolymerization reactions using heterogeneous catalysts. The system, equipped for basic temperature and pressure control, also includes an on-line composition control system which maintains close control of reactor composition for gaseous and liquid monomers as well as for hydrogen. With this apparatus, it is now possible to investigate the fundamental kinetic features of the catalyst system with high precision.