Al-27 MAS NMR spectroscopy has been used to study the surface aluminium compounds formed upon interaction of the supported TiCl4/donor/MgCl2 catalyst with AlEt3 in the presence or absence of the external donor. Similarly with the catalyst without any donors [A.G. Potapov, V.V. Terskikh, V.A. Zakharov, G.D. Bukatov, J. Mol. Catal. A: Chem., 145 (1999) 147], the AlEt2Cl formed adsorbs on the catalyst surface. It was found that there is no influence of internal and/or external donors on the state of aluminium in adsorbed AlEt2Cl in spite of a decrease of the aluminium content in the presence of external donor.