A detailed study of the reactions of phosphorus pentasulfide and Lawesson's reagent with a series of 4,5-bis(RCOCHS)-1,3-dithiole-2-thiones (R-Ph, 4-MeOC6H4, 4-Br C6H4, Me) has been carried out. These reactions lead to fusion of either an unsaturated 1,4-dithiin ring or a thiophene to the dithiole; the former in higher yield, while the latter is a significant product in the reactions with Lawesson's reagent; as well as small amounts of minor products. A mechanistic rationalization of these products is discussed in some detail. The new fused dithioles have been converted to novel series of fused TTF derivatives. (C) 2003 Elsevier Ltd. All rights reserved.