It has been confirmed by UV spectrophotometry and TLC that cytosine and cytidine deaminate to uracil and uridine, respectively, at all pH values. These products slowly degrade further in strongly alkaline solutions. Cytidine also undergoes a parallel degradation in strongly alkaline solutions to a nonchromophoric compound, which can be assigned to a product of pyrimidine ring cleavage. The kinetic expression for the apparent first-order rate constant for the deamination of cytosine and its derivatives is: k1 = (kH2o + koH[OH−])fH2C+ + (koH)1[OH−]fHC, where fH2C+ and fHC are the fractions of substrate protonated and nonprotonated, respectively; and log k-pH profiles were prepared for several temperatures. The microscopic rate constants for deamination and their Arrhenius parameters were obtained. Cytosine showed a pK 'a2 assignable to a dissociation in highly alkaline solutions, and this anionic form appeared resistant to deamination. No such kinetic pK 'a2 was assignable to cytidine. Buffer catalytic constants for deamination were obtained for acetate and carbonate buffer species.