The reaction of the glutacondialdehyde anion with alkyl and acyl isothiocyanates has been investigated. Whereas alkyl isothiocyanates give rise to 1-alkyl-3-formyl-2 (1H)- pyridinethiones, O-acylated glutacondialdehyde derivatives are formed from acyl isothiocyanates in most cases. The reaction courses may be understood on the basis of the principle of hard and soft acids and bases.