For the purpose of reducing the simultaneous production of insoluble polymers during the bis(imino)pyridyl iron-catalyzed ethylene oligomerization, various modifiers like phenol, anisole, 1-naphthol, 2-naphthol, benzoic acid, cyclohexanol, cyclohexyl carbinol, and benzyl alcohol were employed to modify the co-catalyst methylaluminoxane (MAO) and tune the catalytic behaviors in this work. It was found that phenol could serve as a good polymer-retarding modifier, the phenolic hydroxyl group was responsible for the interactions with MAO. Further increasing the aromatic ring size from phenol to naphthols was beneficial for retarding the polymer formation. Meanwhile, an increase of the acidity of the modifiers would promote the interactions between them and MAO, but also tend to easily deactivate the catalytic system. In addition, the alcohol modifiers with a phenol-like structure were also studied, their deactivation effect was found to be stronger than their polymer-retarding effect, making them difficult to obtain a satisfactory low polymer formation with the catalytic system remaining a high activity. POLYM. ENG. SCI., 59:1010-1016, 2019. (c) 2019 Society of Plastics Engineers