Apparent first-order rate constants, k, for the degradation of arabinosylcytosine in aqueous solutions were determined as a function of pH, temperature, and buffer concentrations. Profiles showing the effect of pH on k were constructed from data obtained in hydrochloric acid and sodium hydroxide and by extrapolation of buffer data to zero buffer concentration. Maximum stability is observed in the neutral pH region where arabinosylcytosine undergoes deamination to arabinosyluracil and 90 potency is maintained for 6.5 months in 0.06 Mphosphate buffer, pH 6.9, at 25". The observed rate constant, k, increases as the pH decreases from 6 to 2.8, where it passes through a maximum value and again decreases as the pH approaches zero. In the pH region 0-6, the loss of substrate is accompanied by the formation of an intermediate which reacts further to yield arabinosyluracil. The profile for the maximum yields of the intermediate as a function of pH resembles the k versus pH profile. Kinetics for deuterium exchange at the H, on arabinosylcytosine and TLC studies characterizing the intermediate were combined with the other kinetic data in developing a proposed mechanism involving 2'-hydroxyl participation and the formation of an intermediate in acid pH. In alkaline solutions the loss of arabinosylcytosine is about 10 times more rapid than in acid and most likely is due to hydrolysis of the pyrimidine ring since no arabinosyluracil UV absorption spectra were detected.