Time-resolved chemically induced dynamic nuclear polarization (TR-CIDNP) is applied to follow transformation of the short-lived neutral guanine radical into a secondary guanine radical by its protonation, presumably at position N7. In the initial step the photoreaction of guanosine-5′-monophosphate (GMP) with triplet excited 3,3′,4,4′-tetracarboxy benzophenone (TCBP) leads to formation of the neutral radical G(−H).. The evidence of the radical conversion is based on the inversion of CIDNP sign for TCBP and GMP protons on the microsecond timescale as a result of the change in magnetic resonance parameters in the pairs of TCBP and GMP radicals due to structural changes of the GMP radical. Acceleration of the CIDNP sign change upon addition of phosphate (proton donor) confirms that the radical transformation responsible for the observed CIDNP kinetics is protonation of the neutral guanine radical with formation of the newly characterized cation radical, (G.+)′. From the full analysis of the pH-dependent CIDNP kinetics, the protonation and deprotonation behaviour is quantitatively characterized, giving pKa=8.0±0.2 of the cation radical (G.+)′.