(E)-1-Aryl-2-(methylsulfinyl)-2-(methylthio)ethene, obtained by the Knoevenagel-type condensation of methyl (methylthio) methyl sulfoxide with the corresponding aromatic aldehyde, was found to react with ethylmagnesium chloride to give (Z)-1-aryl-2-(methylthio)ethene predominantly. Since the reaction of (Z)-1-(methylsulfinyl)-1-(methylthio)-2-phenylethene afforded (E)-1-(methylthio)-2-phenylethene