The methylation reaction of alkyl N-acylthiocarbamates in DMF with dimethyl sulfate in the presence of potassium carbonate takes place with high selectivity affording the S-methylated derivative as the principal reaction product. No isomeric reaction products derived from the N- or the O-methylation were isolable. The new carbonimidothioates 3a-g synthesised were fully characterized by spectroscopic methods. The experimental findings are supported by X-ray and theoretical studies at the B3LYP/6-311G(d.p) level.