Methylation of the coordinated heterocyclic thioamide ligand with dimethyl sulfate is studied. When boiled in dimethyl sulfate, bis(benzimidazole-2-N-phenylcarbothioamidato)copper(II) forms copper(II) complexes containing N-phenylbenzimidazole-2-(S-methylcarboisothioamide). The mechanism of this process is considered, and the reaction products are isolated and identified.