The stereospecific hydrogenolysis of vinylic sulfones by sodium dithionite in a protic medium proceeds by addition of HSO2 to give an intermediate which could be isolated after alkylation in situ to a 1,2-bissulfone. The mechanism is therefore of the β-addition-elimination type. In the case of E-2-benzenesulfonyl-2-butene and ethyl iodide a single crystalline diastereoisomer was obtained and shown