Novel transition-metal-catalyzed reactions of disulfides and diselenides with acetylenes are described. In the presence of tetrakis(triphenylphosphine)palladium(0), [Pd(PPh3)4], the (1) of diaryl disulfides and diselenides to terminal acetylenes 1 takes place stereoselectively to give high yields of (Z)-l,2-bis(arylthio)-l-alkenes 2 and (Z)-l,2-bis(arylseleno)-l-alkenes 3, respectively. A mechanistic proposal includes the following: (1) oxidative addition of (ArY)2 [Y = S, Se] to low-valent palladium species, (2) stereoselective cis-thiopalladation or cis-selenopalladation to acetylenes to form a cis-vinylpalladium intermediate, and (3) reductive elimination of the product with retention of the stereochemistry. When the reaction of diaryl disulfides and diselenides with terminal acetylenes is performed under the pressure of carbon monoxide, the carbonylative addition occurs to afford (Z)-l,3-bis(arylthio)-2-alken-l-ones 4 and (Z*)-l,3-bis(arylseleno)-2-alken-l-ones 5, respectively. Carbon monoxide is regioselectively incorporated on the side of the terminal carbon of the acetylenes.