Adsorption of various internal electron donors (IDS) and coadsorption of these IDS with TiCl4 on activated MgCl2 have been studied by DRIFT and analytical techniques. As a result, the structures of the ID species on the (104) and (110) MgCl2 surfaces were proposed. Based on the v(C=O) band resolution data for the adsorbed ID, it was estimated that the activated MgCl2 surface contains ca. 90% of the five-coordinated Mg cations residing on the (104) surface and ca. 10% of the four-coordinated Mg cations residing on the (110) surface. The size of ID molecule was found to influence its adsorption more significantly on the (104) MgCl2 surface than on the (110) MgCl2 Surface. Coadsorption of the various ID and TiCl4 demonstrated that TiCl4 formed greatly weaker surface complexes than the ID; obviously, upon coadsorption TiCl4 occupies mainly the adsorption sites that are inaccessible for the ID because of steric reasons. (C) 2009 Elsevier Inc. All rights reserved.