One-pot annulation of 5-arylidene-2-thiohydantoins (3a-f) with O,O-diethyl hydrogen dithiophosphate (4) yields 3,5-dihydrothieto[2,3-d]imidazole-2(1H)-thiones (6a-f) which undergo LiI-catalysed facile expansion of their thiete ring with carbon disulfide to afford the corresponding dihydrol[1.3]dithiino[4,5-d]imidazole-2,5(1H)-dithiones (7a-f).