Reactions of tetrafluoropyridazine (1) with gaseous thiocarbonyl difluoride or its liquid trimer, viz. bis(trifluoromethyl) trithiocarbonate, in the presence of caesium fluoride yield, selectively, 3,4,6-trifluoro-5-trifluoromethylthiopyridazine (2) or 3,6-difluoro-4,5-bis(trifluoromethylthio)pyridazine (3) depending on the reactant ratio; with an excess of the CF3S– ion source a small amount of 3-fluoro-4,5,6-tris(trifluoromethylthio)pyridazine (4) was formed. The reactions of tetrafluoropyrimidine (5) proceeded unselectively to give mixtures of 2,4,5-trifluoro-6-trifluoromethylthiopyrimidine (6), 2,4-difluoro-5,6-bis(trifluoromethylthio)pyrimidine (7), 2,5-difluoro-4,6-bis(trifluoromethylthio)pyrimidine (8), and in some cases 5-fluoro-2,4,6-tris(trifluoromethylthio)pyrimidine (9). The new trifluoromethylthio derivatives of pyridazine and pyrimidine have been characterized by b.p., 19F n.m.r., and m.s. data.