Here, we show that two tautomers are produced by the protonation of the guanine-electron adduct. The fate of electron adducts of a variety of substituted guanosines was investigated by radiolytic methods and addressed computationally by means of time-dependent DFT (TD-B3LYP/6-311G**//B1B95/6-31+G**) calculations. The reaction of e(aq)(-) with guanosine and 1-methylguanosine produces two transient species, whereas the reaction with N-2-ethylguanosine and N-2,N-2-diethylguanosine produces only one. The two short-lived intermediates, which show a substantial difference in their UV-visible spectra, are recognized to be two purine tautomers (i.e., iminic 18 and aminic 19 forms). The tautomerization 18 -> 19 occurs with a rate constant of ca. 1.5 x 10(6) s(-1), and theory suggests that it is a water-assisted process.