ineffective, cobalt carbonyl (Co2(CO)8) is an excellent catalyst for carbonylative cyclization of internal alkynes with carbonmonoxide. When Co2(CO)8-catalyzed reactions of internal alkynes with organic thiols are conducted in acetonitrile under 4 MPa pressure of carbonmonoxide, thiolative lactonization of internal alkynes successfully takes place with incorporation of two molecules of CO. This carbonylation