The effect of hydrogen on the kinetics of polymerization of propylene catalysed with a non-supported, heterogeneous Ziegler-Natta catalyst, was studied using an integrated measurement methodology of polymerization rate determination and polymer characterization using size exclusion chromatography, temperature rising elution fractionation, and C-13 nuclear magnetic resonance. It was unequivocally shown that the presence of hydrogen during the polymerization of propylene activates catalytic sites that are dormant when hydrogen is absent, increasing the polymerization rate and broadening the distributions of molecular weight and stereoregularity.