Proton transfer between N and O in the hydrogen-bonded system (H3NHOH2)+ is studied by ab initio molecular orbital methods. Potential energy curves are calculated at the hartree–Fock level using the 4–31G basis set for hydrogen bond lengths R(NO) varying from the equilibrium value of 2.664 to 3.10 Å. Short hydrogen bonds are associated with asymmetric single-well potentials in which the minimum corresponds to the NHO configuration. For longer R(NO) separations, the potential is of double-well form, including both NHO and NHO as minima. It is found that the height of the energy barrier to proton transfer is sensitive to both stretches and bends of the hydrogen bond. Continuous changes in the electron density are monitored at various stages of proton transfer via density difference maps and Mulliken population analyses. The initial loss of density from the proton-accepting molecule during the first half of the transfer is accelerated during the second half. A correlation is drawn between the energetics of transfer in a number of systems and the net charge lost by the proton-acceptor group.