Heterogeneous Ziegler-Natta precatalysts (with phthalate as internal donor) were modified by treatments with various Lewis acids (MCln with M = Ga, Sn, Si, and Sb and n = 3, 4, or 5) before their use in the polymerization of propylene. If performed on previously "detitanated" precatalysts, treatments with SnCl4 and SiCl4 lead to a slight activation but especially to an increase of the tacticity whereas GaCl3 and SbCl5 treatments deactivate the catalyst The modification method applied to conventional unmodified precatalysts gave similar trends A significant increase of tacticity (and/or of T-m) and a narrowing of the molecular weight distribution were observed in the case of SnCl4 and SiCl4 treatments (C) 2010 Wiley Periodicals, Inc J Polym Sci Part A Polym Chem 48. 2631-2635, 2010