A new chemical method is developed for the measurement of the number of active centers in heterogeneous titanium-based Ziegler-Natta catalysts. The method can be applied to polymerization reactions of various alkenes using AlMe3 as a cocatalyst. After a polymerization reaction is completed, the solid phase (a mixture of a catalyst and a polymer) is separated from the liquid phase and thoroughly washed with fresh solvent to remove monomers and the cocatalyst. In the absence of an alkene and cocatalyst, active centers with growing polymer chains undergo the B-hydrogen elimination reaction and generate Ti-H bonds. The washed solid is reslurried in a small quantity of a hydrocarbon and treated with benzoyl chloride which is converted by the Ti-H species into benzaldehyde. After removal of excess benzoyl chloride from the slurry (in a reaction with additional AlMe3), the liquid layer is analyzed by the GC method. The amount of benzaldehyde and its reaction products with AlMe3 is the measure of the concentration of Ti-H bonds derived from active centers. Several examples of the active site measurement are presented for polymerization and copolymerization reactions of ethylene, propylene, and 1-hexene with different catalyst systems.