Various 4-methylthiosydnones were prepared by the reaction of 3-substituted sydnones with DmSO and acetyl chloride, while the reaction with methyl phenyl sulfoxide and acetyl chloride gave 4-chlorosydnones. Treatment of 3-substituted sydnones with DmSO and methyl phenyl sulfoxide together with acetyl perchlorate, followed by refluxing with aqueous potassium chloride gave 4-methylthio- and 4-phenylthiosydnones