Electron-rich aza-aromatic compounds such as indoles and pyrroles are structures of particular interest and importance in organic chemistry. A useful methodology for the regioselective introduction of the sulfenyl group into electron-rich aza-aromatics using S-alkyl- and S-arylthiophthalimides as sulfenylating agents is described. Catalytic amounts of CeCl3·7H2O/NaI are crucial to the promotion of this regioselective carbon–sulfur-bond-forming electrophilic aromatic substitution reaction. The reaction occurred under mild conditions, and the products were obtained in good to excellent yields. The method represents an efficient preparation of sulfenyl aza-aromatics, which are useful intermediates for important organic transformations, due to the great importance of functionalized indoles among natural compounds and pharmaceutical products.