Isothiocyanic acid (HNCS) was observed in Sgr B2 by Frerking, Linke, and Thaddeus in 1979 via three lines of its rotational spectrum observed with the Crawford Hill 7 m telescope (the site is perhaps best known for its role in the 1965 discovery of the cosmic microwave background radiation).

HNCS is a periodic analog of isocyanic acid, HNCO. The other two singly-substituted cases are HNSiO and HPCO, neither of which has been identified to date. The thiocyanate ion, SCN^-, may form in ices the same way that cyanate ion, OCN^-, can form, by a proton transfer to a base such as ammonia.