Gas phase ion processes of phosphine have been studied by theoretical calculations and experimental techniques. AZ, initio quantum chemical calculations have been performed on the ion/molecule reactions starting from P+ in PH3, as they have been observed by ion trapping. P+ gives P2Hn+ (n = 1,2) product ions with loss of H-2 or H in different pathways and also reacts in charge-exchange processes to form PH3+ The energies of transition structures, reaction intermediates, and final products, as well as their geometrical structures have been determined by theoretical methods. The initial step is formation of a tripler P2H3+ adduct of C-3 upsilon symmetry (P-PH3+). A hydrogen atom can either be directly lost from the tetracoordinated phosphorus, or first undergo a shift to the other P atom (H-P(+)-PH2), followed by P-H bond dissociation. Dissociation of H-2 from P2H3+ can also occur from both the initial P-PH3+ and HP+-PH2 species yielding PPH+. The heats of formation of the P2Hn+ ionic species have also been computed and compared with experimental data reported in the literature.