A new procedure for the investigation of initial kinetics in a slurry olefin polymerization with a Ziegler-Natta catalyst has been developed. It is based on the accurate timing of short polymerizations that result from the immediate start of a reaction between a catalyst separated in an oil phase and the remaining components of the system upon their being mixed together. The polymer samples are suitable for C-13 NMR analysis and the number of active sites could be calculated.