Summary 13C-NMR has been used to demonstrate the existence and structure of a bisulfite addition product formed from the reaction of sodium bisulfite with yeast RNA. Specifically, upon treatment of yeast RNA with bisulfite at pH 5.2 the signals from C 5 and C 6 of cytosine and uracil are lost with concomitant appearance of signals corresponding to C 4 and C 5 of the uracil-bisulfite adducts. No spectral evidence for the existence of cytosine-bisulfite adducts is observed, thus indicating that the cytosine moieties of yeast RNA are converted to uracil-bisulfite adducts in the presence of bisulfite at pH 5.2. This would be expected based on the pH profile for cytidine deamination. When the pH is increased to 9.5 RNA is regenerated. The spectrum of this regenerated RNA matches that of untreated RNA with the exception of signals originating from cytosine moieties.