Synthesis, properties and reactions of two isomeric dipyrido-1,4-dithiins of the C-2h and C-2v symmetry are described. Their structure detennination and identification are based on spectroscopic methods (H-1 and C-13 NMR, HETCOR, gHMBC and MS), physical properties (mp and R-f), the 1,4-dithiin ring opening reactions and finally X-Ray analysis. A very unusual type of the Smiles rearrangement (S -> S, the pyridyl group migrates from one sulfur atom to another) during the 1,4-dithiin ring opening with sodium methanethiolate enabling isomerization of dithiin with the C-2h symmetry to dithiin with the C-2v symmetry is found.