AbstractIn various samples of the industrially used antiozonant N‐isopropyl‐N′‐phenyl‐1,4‐phenylenediamine (IPPD, Ib), a typical, intensively coloured impurity was detected, and its structure was determined (IIb). The same compound IIb was prepared independently by an acid catalyzed reaction of 4‐aminodiphenylamine (IV) with N‐isopropyl‐N′‐phenyl‐1,4‐benzoquinonediimine (IIIb). IIb is formed by the same route also in technical IPPD, both in the production process and during storage. In the autoxidation of squalene (rubber model) IIb behaves as an effective antioxidant. The simultaneous presence of IPPD does not produce any antagonistic effect. IIa, which is an analogous impurity of technical N‐(1,3‐dimethylbutyl)‐N′‐phenyl‐1,4‐phenylene‐diamine (HPPD, Ia), behaves as an antioxidant similar to IIb. Thus, with respect to the antioxidant effect, the presence of impurities IIa or IIb