We have constructed ab initio adiabatic and quasidiabatic surfaces for the description of the protonation of NH3. For the diabatic states, we applied a recently developed method which is based on the propagation along the reaction coordinate, starting from the dissociation region, of the maximized nonorthonormal overlap between diabatic states calculated at successive geometries. In agreement with earlier calculations [Kaldor et al., J. Chem. Phys. 90, 6395 (1989)], it was found that the adiabatic surfaces cannot explain this charge-transfer process. On the contrary, a single diabatic potential curve correlates smoothly the ground state of NH4+ with the NH3+H+ limit.