AbstractThe kinetics of Ziegler‐Natta polymerisations can be described in terms of a spectrum of active sites, each with differing propagation rates and stereospecificity. It is therefore necessary to use complementary methods of measuring active centres to get the whole picture. Higher activity in polymerisation of propylene using TiCl3‐ Et2AlCl systems can be achieved by varying the catalyst preparation so that the effective surface area is increased or by removing inhibitors. When Et3Al is used as activatior in place of Et2AlCl higher activity is due to a combination of increase in number of centres per mole TiCl3and of increase in their propagation r