Abstract Benzene- and o-nitrobenzene-selenenyl thiocyanate ( 1 and 2) reacted with diazomethane and several α-diazo esters to afford α-arylseleno isothiocyanates ( 5a–10a) and the isomeric thiocyanates ( 6b–8b) as the major and minor products, respectively. Analogous insertion reactions were observed with benzenesulfenyl thiocyanate ( 3) and benzeneselenenyl selenocyanate ( 4) to furnish phenylthiomethyl isothiocyanate ( 11), ethyl α-phenylthio-α-isothiocyanoacetate ( 12) and phenylselenomethyl isoselenocyanate ( 13), ethyl α-phenylseleno-α-isoselenocyanoacetate ( 14), respectively. N-(Phenylseleno)phthalimide formed insertion products N-(phenylselenomethyl)phthalimide ( 16) and ethyl α-phenylseleno-α-phthalimidoacetate ( 17) with diazomethane and ethyl diazoacetate, but effected elimination to the corresponding vinyl selenides, ethyl α-(phenylseleno)propenoate ( 18) and ethyl 2-phenylseleno-3-methyl-2-butenoate ( 19) when treated with diazo esters containing β-hydrogens. Compound 19 was similarly obtained from benzeneselenenyl iodide and the diazo ester. Attempts to effect selenoxide eliminations in compounds 7a–10a were unsuccessful. A new, efficient preparation of 1 from the metathesis of benzeneselenenyl chloride with trimethylsilyl isothiocyanate was developed.