A new type of electrophilic aromatic substitution reaction, the intracomplex reaction, where the electrophilic substitution occurs within the complex formed by attack of a protonated aromatic compound on a proelectrophile to produce the electrophile, was examined in view of the kinetic and regioselectivity features in the Gattermann-Koch formylation. The results of the kinetic experiments established that the intracomplex reaction appears under restricted conditions. A comparison of the intracomplex reaction with the conventional reaction with regard to regioselectivity revealed that the regioselectivity of the intracomplex reaction reflects the structure and proton-transfer dynamics of the protonated aromatic compound as a reaction precursor.