AbstractIn order to study the promoting effect of polycations on the cobalt(II)phthalocyanine‐tetrasodiumsulphonate (CoPc(NaSO3)4)‐catalysed autoxidation of thiols, it is imperative to know the molecular weight dependence of the polymer. Monodisperse oligomers can often supply a lot of information. To elecidate the mechanism of the promoting effect of 2,4‐ionene, a poly(quaternary ammonium)salt, on the CoPc(NaSO3)4‐catalysed autoxidation of 2‐mercaptoethanol, a series of monodisperse 2,4‐ionene oligomers were prepared. Trimeric 2,4‐ionenes, containing two quaternary ammonium groups separated by four methylene groups, showed a high co‐catalytic activity, and active Co‐aggregates were detected with visible light spectroscopy. The spectroscopic behaviour was in close agreement with that of high molecular 2,4‐ionene/CoPc(NaSO3)4complexes. In order to achieve this aggregation several oligomeric ionenes have to act concertedly. If the distance between the ionic sites is short the trimer acts as a simple salt in stablizing the aggregates. With respect to the catalytic activity the optimum polycation/catalyst ratio, expressed as the N+/Co ratio, decreased with increasing chain lenght and reached a constant level of 50 at a 2,4‐ionene nonamer. For the trimers and pentamers this ratio is affected by the type of end‐group. All synthesized 2,4‐ionene oligomers exhibited excellent co‐catalytic properties at the optimum N+/Co ratio, with maximum turnover frequencies of 4600 mol thiol/(mol Co s), i.e. 50 times higher than those obtained for the polymer‐free system. In the case of monodisperse 2,4‐ionene pentamer with bromo end‐groups, double Michaelis–Menten kinetics were observed, as also was exhibi