A series of Ziegler-Natta catalysts, with and without diisobutylphtalate as an internal donor, were modified with alkylaluminum (triethylaluminum or diethylaluminum chloride) and an external donor (diisopropyldimethoxysilane or methyltrimethoxysilane), in increasing complexity. The resulting catalysts were characterized by X-ray photoelectron spectroscopy and extended X-ray absorption fine structure analysis (EXAFS) and evaluated in propylene polymerization. According to EXAFS oscillation, systems bearing diisobutylphtalate as an internal donor displayed a well-defined pattern, suggesting a more ordered structure around the Ti atom. EXAFS data treatment provided two Ti-Cl distances: similar to 1.96 and 2.5 angstrom. The former was shown to have a linear dependence on the Ti (2p(3/2)) binding energy as the catalyst is modified with TEA or with external donors. Conversely, the longest distance is reduced as Ti (2p(3/2)) binding energy increases with catalyst modification. Correlations involving the catalyst activity, Ti (2p(3/2)) binding energy, and Ti-Cl distances could be denoted. (C) 2008 Wiley Periodicals, Inc.