Numerous reports in recent literature claim that NO binds to aquacobalamine (vitamin B-12a) to produce a nitrosyl complex, and that this reaction plays an important role in various biological processes. A careful study of the system has now revealed that there is no reaction between aquacobalamin and NO. The observed reaction is due to the presence of nitrite impurities in aqueous solutions of NO, which react with aquacobalamin to produce the nitrite complex.