Ionizing radiation on DNA mainly generates one-electron oxidized guanine-cytosine base pair (G(+.):C), and in the present paper we study all possible tautomers of G(+.):C by using ab initio approaches. Our calculations reveal that the tautomeric equilibrium follows a peculiar path, characterized by a stepwise mechanism: first the proton in the central hydrogen bond N1(G)-H1-N3(C) migrates from guanine to cytosine, and then the cytosine cation releases one proton from its amino group. During this second step, water acts as a proton acceptor, localizing the positive charge on one of the water molecules interacting with the guanine radical. In agreement with experimental findings, the computed energy barriers show that the deprotonation of the cytosine cation is the speed-limiting step in the tautomeric equilibrium. The influence of the number of water molecules incorporated in the theoretical model is analyzed in detail. The evolution of electronic properties along the reaction path is also discussed on the basis of partial atomic charges and spin density distributions. This work demonstrates that water indeed plays a crucial role in the tautomeric equilibra of base pairs.