Ab initio calculations at the G2 level have been employed to investigate the ligand-exchange reactions between mono-adducts of the phosphenium ion (e.g., [H(3)N-PH(2)](+)) and simple first- or second-row Lewis bases (e.g., NH(3)). We have found virtually all the reactions to proceed without an intermediate barrier via a bis-adduct of the phosphenium ion with two Lewis bases (e.g., [H(3)N-PH(2)-NH(3)](+)). The ligand-exchange reactions are predicted to be experimentally feasible and the bis-adducts of the phosphenium ion to be experimentally observable in appropriate cases. The energetics of the ligand-exchange reactions may be rationalized using qualitative orbital interaction arguments in terms of the electronegativity of the ligand in the mono-adducts and the donor ability of the reacting Lewis base. Comparisons with previous investigations of corresponding ligand-exchange reactions involving pi-ligands reveal that the electronic characteristics of the bis-adduct resemble those of the transition structures for pi-ligand exchange.