Résumé Mercury (II) thiocyanate Hg (SCN) 2 catalyses thiocyanic acid HSCN addition to unactivated acetylenic compounds R 1-C CR 2. Bonding of the SCN moiety to carbon occurs through sulphur or nitrogen depending upon the influence of R 1 and R 2; vinyl thiocyanates NCS-C (R 1) CH 2 are obtained specifically from terminal acetylenic compounds but some symmetrically disubstituted alkynes may afford vinyl isothiocyanates ...