Abstract TiCl 4 supported on silicon oxide or magnesium halides is widely used as catalyst for olefin polymerization from the gas phase. These catalysts are usually activated with organoaluminum compounds such as triethylaluminum (AlEt 3). In the present study, the interaction mechanism between AlEt 3 and titanium chloride supported on a SiO x layer was investigated using X-ray photoelectron spectroscopy (XPS). A flat conductive Si wafer covered with a thin native SiO x layer, was used as support, in order to eliminate electrostatic charging effects during the XPS measurements and allow a more reliable identification of surface chemical species. It was found that the TiCl 4 vapors react with the sputtered oxide layer mainly through isolated surface Si–OH species leading to formation of Si–O–TiCl 3. There was no evidence of Ti reduction after the interaction with AlEt 3 vapors but only a small downward BE shift of the Ti 2p 3/2 peak. This behavior was attributed to the effect of alkylation of the titanium oxychloride species. On the basis of the present results it is proposed that the alkylation takes place by means of surface complex formation with AlEt x that may or may not be chemically bonded on the SiO x surface.