Reaction of 2-amino-3-nitropyridine with acid chlorides in the presence of dimethyl sulfoxide and a tertiary base, such as pyridine, unexpectedly yielded the aminal N,N′-bis(3-nitro-2-pyridinylimino)methylene 5. A similar aminal 6 formed when 2-amino-5-nitropyridine was treated under the same conditions. However, simple aminopyridines such as 2-aminopyridine or 2-amino-6-methylpyridine gave only the 2-N-methylenethiomethyl derivatives. It was proved that the aminals are formed by the attack of a suitable base on the N-methylenethiomethyl intermediates with expulsion of methyl mercaptan. The formation of these unusual aminals may be explained to be taking place via a Pummerer type rearrangement.