The interaction of AlEt3 cocatalyst with the internal donor (ethyl benzoate-EB) in the supported Ziegler-Natta (TiCl4/EB/MgCl2) catalysts and the EB/MgCl2 support was studied by DRIFT spectroscopy. For EB/MgCl2 samples, it was shown that AlEt3 firstly removed the more weak surface complexes of EB from the MgCl2 surface and competed with EB at adsorbtion for releasing sites of the MgCl2 surface. EB, as the external donor, keeps constant the initial content and distribution of surface complexes of EB at the interaction of the AlEt3/EB mixture with the EB/MgCl2 support. At the interaction of AlEt3 with the TiCl4/EB/MgCl2 catalysts one type of surface complexes of EB is mainly removed. A part of removed EB adsorbs again on the catalyst surface in the form of complexes with aluminum-containing compounds. At the interaction of the catalyst with the AlEt3/EB mixture the EB content in the catalyst changes slightly, but the distribution of different surface complexes of EB in the catalyst changes noticeably compared to that in the initial catalyst. EB, as the external donor, adsorbs both on surface MgCl2 sites released at the removal of the internal donor and oil aluminum-containing Compounds adsorbed on the catalyst surface. (C) 2008 Elsevier B.V. All rights reserved.