The development of new high activity catalysts has highlighted the need for improved methods for the determination of active centre concentrations in Ziegler-Natta catalysis. Although a number of methods have been used by various researchers (1,2) very little work on the comparative evaluation of these methods has been carried out (3,4,5). The present work concerns some aspects of inhibition and kinetic studies for the polymerization of propylene and quenching and radio-tagging studies for the polymerization of 4-methyl pentene—1.