The nucleophilic addition of thiocarbamate imidazoliumylide to aldehyde triggered sequential intramolecular N to O migration of thiocarbonyl amide group and reversible oxygen–sulfur rearrangement to afford 2-imidazolium alkylcarbamothioate. The ortho group on phenyl of aldehyde strongly affects the balance of the O- to S-rearrangement.