The pH-dependence of the 3H to 1H exchange between water and C(8)H groups of purine, adenine, 9-methyladenine, 7-methyladenine, hypoxanthine, guanine, xanthine as well as C(2)H groups of imidazole and benzimidazole was studied. It was shown that within the pH-ranges, where the majority of molecules under study are non-ionized, the values of observed rate constant (kobs) do not depend on pH. Beyond these ranges the values of k(obs) are increased or decreased depending on the type of ionizaiton of the compound under study in appropriate pH range. The observed pH dependence of the 3H to 1H exchange is in a good quantitative agreement with ylide mechanism of the exchange reaction. According to this mechanism the 3H--1H exchange takes place in N(7)-protonated forms of the purine derivatives and in zwitterions with positive charge on N(7). The ylide mechanism of the exchange reaction is also suggested by the fact that the true exchange rate constants (k+) of protonated forms of the studied compounds, calculated from the values of k(ods), rises linearly with the increase of their protonation constant (Ka1)--the tenfold increase of Ka1 leads to about four-fold rise of k+. The knowledge of 1h to 3H exchange mechanism in C(8)H groups of purine derivatives allows to estimate alterations of reactivity of the purine residues in polynucleotides and nucleic acids depending on their conformation.