Titanium-magnesium Ziegler-Natta catalysts are the basis of the modern processes for polypropylene production. These catalysts have been the object of numerous studies for many decades. Nevertheless, the structure of the catalysts remains a significant research focus. One of the major questions to be answered is the nature of the active sites precursor − the titanium compound, its form and structure on the surface of the catalysts. In the present study, the experimental data show that the precursor of the catalyst active site is TiCl4, which is weakly bound to the surface of the catalyst. This TiCl4 is able to migrate outside the catalyst particles and to adsorb on activated MgCl2, giving rise to new precursors of the active sites. The quantity of TiCl4, which is able to migrate easily, amounts to about half the total amount of titanium compounds in the catalysts. At the same time, the rest of titanium is tightly bound to the catalyst surface and does not produce the active sites.