The results of the search for new organoselenium reagents and their utilization in organic synthesis are described herein. They include 1) phenylselenenyl chloride in acetonitrile/water (hydroxyselenation of olefins), 2) phenylselenenyl chloride/trifluoromethanesulphonic acid/water in acetonitrile (amidoselenation of olefins), 3) phenylselenenyl chloride/mercury (II) thiocyanate [selective synthesis of β- (phenylseleno) alkyl isothiocyanates], 4) phenylselenenyl bromide (intramolecular amidoselenation of N-alkenyl amides), 5) phenylselenenyl iodide (carbon-carbon bond formation in diolefins), 6) 2-pyridylselenenyl bromide (2-pyridylseleno group is a better leaving group than phenylseleno group in selenoxide elimination), 7) phenyl selenocyanate/copper or nickel halide/alcohol (oxyselenation of olefins), and 8) potassium selenocyanate/copper (II) halide (selenium-containing heterocycles from diolefins) and related reactions.