Iodine monochloride in aqueous sulfuric acid turned out to be a convenient and general reagent for preparative iodination of alkylbenzenes, phenol ethers, and aromatic amines. The relative selectivity and activity of iodine monochloride in aqueous solutions of sulfuric acid with various concentrations were determined using toluene as model substrate. Raising the sulfuric acid concentration results in considerable increase of the electrophilicity of ICl. Effective sulfuric acid concentrations were determined for specific substrate series. Iodine monochloride in aqueous sulfuric acid shows enhanced selectivity in the synthesis of monoiodo derivatives.