Kinetic studies of the Diels–Alder cycloadditions of a series of trans-stilbenes (2) to 2,3-dimethylbuta-1,3-diene (3), catalysed by tris(4-bromophenyl)aminium hexachloroantimonate (1˙+), reveal a curved Hammett–Brown plot, consistent with a mechanistic transition from reversible to rate determining ionization of 2 to 2˙+ as the substituents become progressively more electron donating. Further, substituent