To examine the effect of neighboring group on thermal treatment of O-alkyl S-methyl dithiocarbonates, syntheses of O-ethyl S-methyl dithiocarbonates holding alkyl (or aryl)-sulfinyl group were tried by the reaction of 2-alkyl (or aryl) sulfinylethanol and carbon disulfide in an aqueous sodium hydroxide followed by methylation with methyl iodide. However, the reaction did not progress as expected and gave 2-alkyl (or aryl) sulfinylethyl methyl trithiocarbonates (Va, -d) as main products. Nuclear magnetic resonance spectral data of these products suggested the formation of trithiocarbonate to occur via elimination and addition. It was thereby clarified that the alkyl (or aryl) sulfinyl as the neighboring group worked as an electron-attracting group to favor elimination.