A metal-free p-toluenesulfonic acid (TsOH·H2O) mediated synthesis of 3-ketoquinolines from anilines, enaminones and DMSO has been developed. In this transformation, DMSO was activated by TsOH·H2O and provided the one-carbon unit of the 3-ketoquinolines. A plausible mechanism involving an electrophilic sulfeniumion intermediate was proposed.