The action of thallium(III) acetate on a number of indoles, differently substituted in the 2- and 3-positions, has been investigated. Monomeric and 'dimeric' oxidation products are obtained; the structures of the products depend on the substituents present in the substrate. A novel oxidative dimerisation of 2,3-disubstituted indoles to indolo[3,2-b]carbazoles is observed. The generation of the products could be rationalised by the initial formation of an unstable, non-isolable 3-thallated-indolenine derivative.