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Methylations of heterocyclic systems, such as benzimidazole, naphth[2,3-c]imidazole, imidazo[4,5-c]-pyridine, purine, pyridine, pyrimidine, pyridazine and s-triazolo[4,3-b]pyridazine, which bore SH, NH and/or OH groups, were carried out with dimethylformamide dimethyl acetal to give the corresponding S-, N- and/or O-methyl derivatives in high yields. Selective methylation of some compounds containing both SH and NH groups took place to give first the S-methyl and subsequently the S,N-dimethyl derivatives. No side reactions, such as C-methylation, were observed.