Important aspects of the electrochemical reduction of 4-iodophenyl sulfonyl phthalimide and 4-iodophenyl sulfonyl chloride are investigated in acetonitrile at a glassy carbon electrode. A change in both the electron transfer mechanism as well as in the global reduction process is observed upon changing the group attached to the 4-iodophenyl sulfonyl moiety. For 4-iodophenyl sulfonyl phthalimide, the initial ET follows a stepwise process involving the intermediacy of the corresponding radical anion, which is readily detected by increasing the scan rate to 100 V/s. With 4-iodophenyl sulfonyl chloride, a concerted ET mechanism, where the S-Cl chemical bond is cleaved upon injection of the first electron, is followed. For the latter compound, an interesting autocatalytic mechanism is encountered, where the 4-iodophenyl sulfonyl chloride is reduced both at the electrode and through homogeneous electron transfer from the resulting sulfinate anion. An electron transfer between the generated sulfinate anion and the parent molecule is the basis to the observed autocatalytic process. Consequently, the reduction process depends on both the concentration of the substrate and the scan rate. Theoretical calculations results help to rationalize the observed difference in the first electron transfer mechanism. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.102112jes] All rights reserved.