The regioselective oxyselenenylation of terminal olefins or cycloalkenes was achieved via olefin oxymercuration and subsequent radical substitution by selenenylating reagent such as S-benzoyl Se-phenyl selenosulfide, diphenyl diselenide, or phenylselenocyanide. Successful application to intramolecular selenocyclization of a mercurial bearing an α,β-unsaturated ester is also described.