Earlier experimental investigations of inelastic and charge-transfer scattering of protons by water molecules at collision energy up to about 50 eV (Friedrich et al. J. Chem. Phys. 1987, 87, 5256) are examined via an accurate computational study of several features of various ground- and excited-state electronic potential energy surfaces (PES’s). Highly correlated calculations of representative ’cuts’ of the multidimensional PES’s are able to show the preferential dynamical coupling to either bending or stretching vibrational motions depending on the direction of proton approach and on the existence of curve-crossing regions leading to nonadiabatic charge-transfer processes, in general agreement with experiments. The general implications of our results on low-energy ion-molecule reactions are discussed for the H3O+ system, for which a rather detailed analysis of valence forces and of unimolecular breakup pathways had been Previously carried out in our group (Gianturco, F. A.; et al. J. Chem. Phys. 1994, 101, 3952).