Three diethers, 2,2-diisobutyl-1,3-dimethoxypropane, 2,2-dicyclopentyl-1,3-dimethoxypropane, and 2-ethyl-2-butyl-1,3-dimethoxypropane, differing in the bulkiness of the alkyl substituents in position 2, have been used as internal donors in MgCl2-TiCl4-diether/AlR(3) catalysts for the polymerization of propylene. The performance of the three diether-based catalysts is compared with that of a "traditional" MgCl2-supported catalyst based on the phthalate/silane donor pair and with that of a "donor-free" catalyst by coupling the temperature rising elution fractionation (TREF) analysis with a study of the stereoregularity of the first monomeric unit insertion. On the basis of the experimental data obtained, the action mechanisms of the different donors are discussed taking into account the various models presented in the literature.