The aluminum chloride (AlCl 3 )-facilitated electrophilic aromatic substitution (EAS) reaction of benzene (PhH) with thionyl chloride (SOCl 2 ) in the molar ratio of PhH:SOCl 2 :AlCl 3 = 2:1:1 was studied under different conditions. At about 70 • C, adding granular AlCl 3 (equimolar to SOCl 2 ) piecewise to a 2:1 mixture of PhH and SOCl 2 led to formation of a highly pure (99.9%) sole product of diphenyl sulfoxide (Ph 2 SO) in an isolated yield of 85% after aqueous workup. It represents a simple and very efficient synthesis of Ph 2 SO, an important fundamental organic reagent. At about 0 • C, adding PhH (twice the molar quantity of SOCl 2 ) to a 1:1 mixture of SOCl 2 and AlCl 3 gave both diphenyl sulfide Ph 2 S (yield: 37%) and Ph 2 SO (yield: 27%) after aqueous workup, together with a small amount of S-phenyl benzenesulfonothioate PhSO 2 SPh (yield: about 5%) and a trace amount of diphenyl sulfone (Ph 2 SO 2 ). A possible mechanism has been proposed to account for formation of all these products, especially for the formation of Ph 2 S [S (II)] from SOCl 2 [S (IV)], which involves a novel reduction of sulfur in the course of the EAS reaction.