The gas-phase ion chemistry of phosphine has been investigated by ab initio theoretical calculations and experimental techniques. Following a previous study of H and H-2 loss pathways from the P-3-PH3+ adduct (generated by P-3(+) reacting with PH3), the quantum chemical study of these processes has been extended to the ion-molecule reactions starting from (PH+)-P-2 reacting with PH3, as observed by ion trapping. In these experiments, PH+ reacts to give P2Hn+ (n = 2,3) product ions, with loss of H-2 or H in different pathways, and also reacts in charge-exchange processes to form PH and PH3+. Moreover, elimination of two hydrogen molecules has been observed leading to the formation of the P-2(+) ion species. All these processes take place at similar rates, their constants ranging from 1.2 to 5.5 x 10(-10) cm(3) molecule(-1) s(-1). The geometrical structures and energies of transition structures, reaction intermediates, and final products have been determined by ab initio theoretical methods. The initial step is formation of the (HP)-H-2-PH3+ adduct. Then, a hydrogen atom can be directly lost either from dicoordinated or tetracoordinated phosphorus, to give P-3-PH3+ or (HP)-H-1=PH2+, respectively. Alternatively, one hydrogen can first undergo a displacement from the latter to the former P atom to give (H2P)-H-2-PH2+. This migration can then be followed by P-H bond dissociation, yielding again (HP)-H-1=PH2+. Dissociation of H-2 can also occur, from either the initial HPPH3+ or rearranged H2P-PH2+ isomeric ions, yielding the (HP)-H-2=PH+ or (H2P)-H-2=P+ tons, respectively. These last species are related by a H-migration process. A last H-2 loss from H2P = P+ produces P-2(2)+. Other pathways were explored, but proved not to be viable. The heats of formation of the P2Hn+ (n = 0-4) ionic species have also been computed and reported with the experimental data in the literature.