Abstract It has long been noted that the H 5 proton of cytosine and its related derivatives exhibits unusually broad resonances in the pmr spectrum under certain conditions of temperature and solution pD. We have examined this phenomenon as a function of temperature, solution pD as well as the external magnetic field, and have shown that the line broadening arises from chemical exchange between the amino and imino tautomers of the cytosine base. The observation of sharp H 5 resonances in cytosine derivatives existing in only the amino or the imino tautomeric structure supports this interpretation. The imino tautomer was estimated to be present to the extent of 15 ± 3% at room temperature in neutral equeous solution.