Substituent effects of an N-amino group on the H-D exchange of ring hydrogens of adenines were compared with those of N-methyl group by treating N3-, N7-, and N9-substituted adenines in phosphate buffered D 2 O media at 70°C. N-Aminoadenines underwent H-D exchange in position 8 faster than the corresponding N-methyladenines. Rates of the exchange of C8-hydrogen of N9-substituted adenines were independent of pD of the medium in an ordinary pD range above their pKa. In strongly alkaline media, N9-aminoadenine underwent H-D exchange at position 2, but N9-methyladenine did not.