The article describes chemical reactions between a fourth-generation solid Ziegler-Natta catalyst for propylene polymerization, TiCl4/MgCl2/diisobutyl phthalate (DIBP), and cocatalyst mixtures containing AlEt3 and an external donor compound, (Cy)(Me)Si(OMe)(2). The solid catalyst component contains several surface complexes of diisobutyl phthalate with MgCl2 and TiCl4, as well as complexes Of MgCl2 and o-phthaloyl chloride, which is formed in a reaction between DIBP and TiCl4. When the solid catalyst is contacted with the cocatalyst mixture, all these adsorbed carbonyl species react with AlEt3. The reactions reduce the carbonyl groups of DIBP and lead to the formation of various dialkylaluminum alkoxides. The order of reactivity of the complexes in the reactions with AlEt3 is: phthaloyl chloride/MgCl2 > DIBP/MgCl2 approximate to DIBP/TiCl4. These reactions result in a complete removal of phthaloyl chloride from the catalyst surface, in a significant reduction of the total content of the MgCl2/DIBP and TiCl4/DIBP complexes, and in the reduction of surface Ti-IV species to Ti-III. The second component of the cocatalyst mixture, complexes of(Cy)(Me)Si(OMe)2 with AlEt3, are strongly adsorbed on the surface of the solid catalyst and on the surface of the products of its reactions with excess AlEt3. The most probable coordination site for the silane species is surface Ti atoms, including the active centers in olefin polymerization reactions. (c) 2008 Elsevier B.V. All rights reserved.