Ortho-carboxyl-substitution increases the acidity of hypervalent diaryliodonium salts and makes them useful catalysts in the synthesis of bis(indolyl)methanes under mild conditions, with yields up to 98%. The halogen bond-assisted hydrogen bonding was demonstrated by crystal structure analysis. The interaction distance of 2.619 Å between I(III) and the carboxyl O atom indicates either a long covalent or a very short halogen bond; analysis of the electron density shows the latter description is appropriate.