The first two vertical ionization energies of H2O-H2O, NH3-H2O, and FH-H2O hydrogen-bonded complexes and the rearrangement processes in their ionized states have been studied by ab initio calculations that include correlation energy. In all cases, the two lowest ionic states are the 2A' and2A" states depending on whether the ionization is located in the proton acceptor or in the proton donor molecule, respectively. If the ionization is produced in the proton donor, the 2A" state, the dimer evolves to a proton-transfer complex. However, if the ionization is produced in the proton acceptor, the 2A' state, the dimer rearranges to a non-proton-transfer hydrogen-bonded structure in which the role of the two monomers is just the inverse of that of the original neutral dimer. For the water-water and water-ammonia dimers, relaxation of the 2A' state can also lead to three-electron hemibond complexes. © 1994, American Chemical Society. All rights reserved.