1. A bisulfite-O-methylhydroxylamine mixture under mild conditions causes the quantitative conversion of cytidine monomers into an equal amount of two diastereomeric N4-methoxy-5,6-dihydrocytidine-6-sulfonate derivatives. The reaction also occurs for cytidine residues in oligonucleotides and denatured DNA. Other usual nucleoside residues remain intact under the reaction conditions. 2. The modified residues are stable upon acid and alkaline hydrolysis of RNA; but are converted to a mixture of uracyl and N4-methoxycytosine by perchloric acid at 100°C; the treatment with 66% formic acid and 2% diphenylamine (conditions of DNA depurinisation) results in the splitting of the N-glycosidic bond of the modified deoxycytidine residue. 3. The internucleotide bond in modified G-Cp is resistant to snake venom phosphodiesterase in the presence of phosphomonoesterase and can be only slowly split by guanyl ribonucleases. 4. The reaction mechanism and its application in the study of nucleic acid structure and function are discussed. © 1974.