Although there was a tentative detection of pentacarbon monosulfide (C₅S) in 1993 by Bell et al., the best evidence for its detection to date is the 2014 report by Agúndez et al. Both reported detections were made toward the carbon-rich source IRC +10216, a well-studied circumstellar shell around CW Leo. The most recent observations were made with the IRAM 30m telescope.

Pentacarbon monosulfide or 5-thioxo-1,2,3,4-pentatetraenylidene (according to Landolt-Börnstein) is not stable enough to exist for very long in the laboratory. Its rotational spectrum was characterized by Gordon et al. and various vibrational features were measured by Szczepanski et al. and Thorwirth et al.