The gas phase ion chemistry of phosphine has been investigated by ab initio theoretical calculations and experimental techniques. Following previous studies dealing with P-3(+) and PH+ reacting with PH3, the quantum chemical study of these processes has been extended to the ion/molecule reactions starting from PH2+ and PH3 (reaction a) or PH3+ and PH3 (reaction b), as observed by ion trapping. In these experiments, PH2+ reacts to give P2Hn+ (n = 1,3) product ions, with loss of H-2 through different pathways. These processes take place at quite different rates, their constants being 2.6 and 7.6x10(-10) cm(3) molecule(-1) s(-1), respectively. The geometrical structures and energies of transition structures, reaction intermediates, and final products have been determined by ab initio theoretical methods. The initial step of the reaction of PH2+ with PH3 is formation of the H2P-PH3+ adduct. Then, a hydrogen molecule can be directly lost either from tricoordinated or tetracoordinated phosphorus, to give P-PH3+ or HP = PH2+, respectively. The shift of one H atom in HP = PH2+ produces the bridged HP(H)PH+ ion, from which further dissociation of H-2 yields PPH+. The initial step of the reaction of PH3+ with PH3 is formation of the H3P-PH3+ adduct. Then inversion of the H atoms in the PH3 group transforms the adduct in an electrostatic complex. This last species is related by a dissociation process to the PH2 and PH4+ products. The heats of formation of the P2Hn+ (n = 1-6) ionic species have been computed and compared with the experimental data in the literature.