Summary: The stereoselective reduction of acetylenic sulfones to cis vinylic sulfones was effected by using a novel reduction system consisting of copper (II) salts (Cu (BF,),, Cu (OTf),, CuF,, etc.) and a hydrosilane. The best result was obtained by the use of Cu (BF,),. A divalent copper hydride is proposed to be responsible for this reduction.