The state of internal donor (ID) in the supported titanium-magnesium (TiCl4/ID/MgCl2) catalysts for stereospecific propylene polymerization has been studied by diffuse reflectance infrared spectroscopy (DRIFT). The samples were prepared via interaction of highly dispersed MgCl2 with different IDs (ethyl benzoate, EB; di-n-butyl phthalate, DBP) and TiCl4. It was found that the DRIFT spectra of carbonyl groups of IDs adsorbed on MgCl2 could be best described as a superposition of several overlapping vibration bands from a variety of surface complexes. Within this model the content of individual EB and DBP complexes on the MgCl2, surface was calculated. In the case of EB, three main complexes were found on the MgCl2 support in about equal proportions. In the case of DBP, only one of three complexes was preferentially formed. The surface content of both EB and DBP was found to decrease in presence of TiCl4. At the same time TiCl4 had influenced the distribution pattern of EB but not DBP complexes. The most likely scenario of competitive adsorption of TiCl4 and IDs on the MgCl2 support is discussed. A model describing surface distribution of TiCl4 on MgCl2 is proposed for TiC4/EB/MgCl2 and TiCl4/DBP/MgCl2 catalysts. (c) 2005 Elsevier B.V. All rights reserved.