The energetics, geometrical structures, ionization process, dissociation channels of pyridine-ammonia (Py... NH3 and the corresponding cation complexes are investigated with ab initio theory. Vertical ionization potentials (vIPs) were calculated at the spin projection UMP2/6-31G(d,p) (PUMP2/6-31G(d,p)) level. During the relaxation process, the vertically ionized complex can gain some excess energy, which makes relevant dissociation channels favorable. The potential energy surface of the rearrangement processes and the dissociation channels were calculated at B3LYP/6-31G(d,p)//R(U)HF/6-31G(d,p) level, showing that when the Py... NH3 is vertically ionized, the dissociation into PyH+ and NH2 is the dominant channel. Also, the rearrangement processes (proton transfer) were investigated in detail by moving the proton along the hydrogen bond. (C) 2004 Elsevier B.V. All rights reserved.