The chemical interaction between the procatalyst MgCl2/TiCl4/EB (EB = ethylbenzoate) and the cocatalyst triethylaluminium was investigated with the purpose to find a relationship between the degree of titanium reduction and the catalytic activity in ethene and propene polymerizations. This interaction was studied in dependence on Al/Ti molar ratio, activation temperature and aging time by measuring the changes of titanium oxidation states during either aging time and polymerization course. Ti(III) and Ti(II) were determined by two redox titrations, while Ti(IV) was calculated by difference, the total titanium amount being known. It was found that Ti(IV) undergoes reduction to lower oxidation states owing to treatment with AIEt(3) and this process is dependent on activation temperature, aging time and Al/Ti molar ratio. However, in contrast to literature reports, the Ti(IV) reduction appears to be incomplete also by using an excess of cocatalyst AlEt3 or long aging times. Thus, a catalyst activated for 2h at 70 degreesC with a molar ratio Al/Ti = 200, showed the following distribution: Ti(IV):Ti(m):Ti(II) = 9:31:60. The activated catalysts were tested in the ethene and propene polymerizations carried out under the same conditions and the obtained activities were compared with the variation of the titanium oxidation state distribution. Whereas both Ti(III) and Ti(II) species appear to be active in the ethene polymerization, in the case of propene only the species in the higher oxidation states, such as Ti(III) and possibly Ti(IV), are catalitycally active. In particular, the variation of the catalyst activity in the propene polymerization is consistently parallel with the variation of the Ti(III) species. Finally, in order to verify the possible influence of free monomer on the titanium reduction process, the changes in titanium oxidation states during the course of l-hexene polymerization in heptane solution were measured.