Ultraviolet spectra show that hypoxanthine and 6‐thiopurine are present in aqueous solution predominantly as 1H, 9H‐tautomers. However, a 3‐methyl group causes the imidazole ring to assume the 7‐NH form. The sequence of anion formation in hypoxanthine is 9(7) → 1 and in 6‐thiopurine 1 → 9(7). The NMR spectra suggest that the mono‐anion of hypoxanthine is a mixture of tautomers. Protonation always takes place in the imidazole ring, with the exception of 3‐methyl derivatives. In the latter, formation of mono‐cations involves the pyrimidine moiety.