The discovery of thioformyl cation (HCS⁺ or protonated carbon monosulfide) was reported by Thaddeus, Guélin, & Linke in 1981 based on observations over several years in Orion A, the DR 21 region in Cygnus, and Sgr B2 using various telescopes, including the Bell Labs Holmdel Horn 7 m telescope in Crawford Hills, New Jersey. Experimental detection of the observed J=1-2 rotational transition was reported concurrently by Gudeman and coworkers, the group of Claude Woods. What began as the detection of an apparent triplet was recognized as three different interstellar species (also including HOCO⁺).

Three other lines assigned tentatively to HCS⁺ reported in the initial astronomical detection were not definitely identified until Bogey and coworkers measured the microwave spectrum through the J=7-6 transition.

HCS⁺ was detected in the extragalactic source PKS 1830-211 in 2013 by Muller and coworkers.