A highly chemo-, regio-, and stereoselective method for the synthesis of (Z)-vinylic selenosulfides and (Z)-vinylic tellurosulfides in a one-pot reaction of terminal alkynes, diaryl disulfides, and diaryl diselenides (ditellurides) catalyzed by simple base cesium hydroxide monohydrate is described. Due to the different activities of the carbon-chalcogen bonds, the target products cleave selectively and act as a kind of readily available platform molecule for the synthesis of tetrasubstituted alkenes. The mechanism of thioselenation was studied by experimental and theoretical methods.