AbstractStudies of the effects of ring substituents in aromatic amineN‐oxide or amine modifiers on Ziegler‐type propylene polymerization catalyst performance are reported. Among sterically hindered amines orN‐oxides, theo,o‐dimethyl substituted derivatives provide the best stereospecificity with little loss in activity. On the other hand,m‐ orp‐substituted derivatives with electron‐withdrawing substituents were found to activate the catalyst, producing higher yields relative to the unmodified (TiCl3‐Al alkyl) catalyst system. The inductive and steric effects introduced by the modifiers in the catalyst system are discussed. Aromatic amineN‐oxides generally produce less active catalyst systems relative to their parent amines, owing presumably to a redox reaction between the N—