The reactions of 23 transition metal ions M-(g)(+) with phosphane (PH3) have been investigated using Fourier transform ion cyclotron resonance spectrometry. Two main reaction pathways are observed. sequential dehydrogenation of multiple (up to nine) phosphane molecules, and addition of multiple (up to four) phosphane molecules. The addition and dehydrogenation reaction pathways are, for the most part, mutually exclusive for a particular metal. Dehydrogenation of phosphane is observed for metal ions toward the lower left of the transition block, and the addition process is predominant toward the top right. The dehydrogenation process is considerably faster than the addition, and sequentially produces the species MPH+, MP2+, MP3H+, MP4+, etc. Density functional calculations were used to evaluate collision trajectories, reaction mechanisms, structures of intermediates (including a number of MoPxHy+ species), and to identify the factors determining whether each metal reacts by dehydrogenation or addition. Insertion of M+ into P-H bonds is calculated to be a barrierless, exergonic process for only those metals which are observed to dehydrogenate phosphane molecules. A reaction mechanism involving the transfer of H atoms from P to M followed by the elimination of H-2 from M is profiled for Ru. The energetic origins of the metal influence on reaction type have been traced to the energy changes for the first stage migration of H from P to M. These experimental and theoretical results should be valuable in applications where PH3,,, is used in the generation of bulk and surface materials involving metal phosphides.