Ab initio SCF and SCF-CI calculations with the STO-3G basis set have been performed to investigate the structures and energies of water-cytosine complexes and the intermolecular water-cytosine surface in the cytosine molecular plane. Although there are six nominal hydrogen-bonding sites in this plane, only three dimers are distinguishable in the ground state. The most stable has an energy of -10.7 kcal/ mol, and is found in the N1-H and 0 2 region. An asymmetric cyclic structure in which the water molecule bridges adjacent N1-H and 0 2 sites is the preferred form of this dimer. The dimer in the region between 0 2 and N4-H' of the amino group is slightly less stable at -10.4 kcal/mol, and also has an asymmetric cyclic structure as the preferred structure, with the water molecule bridging amino N4-H' and N3 hydrogen-bonding sites. The third dimer has the amino group as the proton donor to water through the hydrogen cis to Cg, and a stabilization energy of -7.0 kcal/mol. The water-cytosine surface is characterized by deeper minima and higher barriers than the water-thymine surface and by a decreased mobility of the water molecule between adjacent hydrogen-bonding sites. Absorption of energy by the C2=0 group leads to the first nx* excited state in which interactions of water with 0 2 are broken. The water-cytosine dimers remain bound in this state, but may change structurally. In the second nx* state interactions between water and N3 are no longer stabilizing. As a result, the dimer in the 0 2 and N4-H' region collapses to either a dimer with water the proton donor to 02, or one with N4-H' the proton donor to water. The other two dimers remain bound. All excited dimers are destabilized on vertical excitation relative to the ground state.