The acylation of 2-amino-l-methylbenzimidazole in acetone proceeds with the formation of 2-amino-3-acyl-l-methylbenzimidazolium salts. When these salts are heated, they rearrange to 2-acylamino derivatives, but they react with bases to give 2-imino-3-acyl-l-methylbenzimidazolines, which are also readily rearranged to 2-acylamino derivatives. The rearrangement of the aroyl derivatives is facilitated when there are acceptor substituents in the aroyl residue.