Amino proton exchange with water could be a useful probe of nucleic acid structure and kinetics. This requires an understanding of exchange processes in the mononucleosides, which has been only partial up till now. We investigate these processes by nuclear magnetization transfer in cytidine, cAMP, cGMP, and endocyclic-methylated derivatives. From the effect of pH on the exchange rate, we disentangle the exchange processes which involve the neutral nucleoside or the nucleoside protonated on an endocyclic nitrogen, and those which operate by basic catalysis (proton transfer to OH- or to water) or by acid catalysis (proton transfer from hydronium). (Cyclic AMP) At basic pH, the amino proton transfers from the neutral nucleoside to OH-. Between pH 7 and pH 5, it transfers from the endocyclic-protonated nucleoside to water. Endocyclic protonation lowers the deprotonation pK of the amino group by about 9 units. This is confirmed by the effect of proton accepters such as ammonia on amino proton exchange. Below pH 5, another process sets in. By reference to proton exchange in primary amides, we identify it tentatively as acid catalysis by transfer from hydronium to the amino group of the neutral nucleoside. A protonation pK of -6 is derived. (Cytidine) At basic pH, amino proton exchange proceeds by transfer from the neutral nucleoside to OH-. Below pH 6, the amino proton transfers from endocyclic-protonated cytidine to OH- (pH > 4) or to water (pH < 4). As in cyclic AMP, endocyclic protonation lowers the deprotonation pK of the amino group by about 9 units. Acid catalysis is not observed. (Cyclic GMP) At basic pH, the amino proton transfers from the neutral nucleoside to OH-. Endocyclic protonation (at N7) does not accelerate exchange. This indicates that it has only a weak effect on the deprotonation pK of the amino group, an explanation supported by the lack of an effect of proton accepters such as ammonia on amino proton exchange. Below pH 6, exchange is acid-catalyzed as it is in cAMP. A protonation pK of -6.3 is derived. Similar processes occur in the methylated derivatives. In 7-methylguanosine, there are two base-catalyzed and two acid-catalyzed exchange processes. In all cases, a quantitative description of the exchange rates is obtained. Application to base-paired oligonucleotides is discussed briefly.