The thiourea/squarate/tetraethylammonium adduct and the thiourea/croconate adduct were investigated by Raman spectroscopy. The former had been previously investigated by X-ray crystallography which revealed an extended network built from squarate and thiourea, connected by hydrogen bonding. The assignment of the vibrational modes in the thiourea/squarate adduct was facilitated by the preparation of the species containing deuterated thiourea. The vibrational shifts observed in the spectrum of the adducts indicate that the involved hydrogen bonds are only of moderate strength. In addition, the well-known Jahn–Teller pattern present in the Raman spectrum of oxocarbon anions is also observed to a great extent in the spectra of their thiourea adducts. Copyright © 2004 John Wiley & Sons, Ltd.