Gas-phase chemistry of bare and oxo-ligated protactinium ions has been studied for the first time. Comparisons were made with thorium, uranium, and neptunium ion chemistry to further the systematic understanding of 5f elements. The rates of oxidation of Pa+ and PaO+ by ethylene oxide compared with those of the homologous uranium ions indicate that the first and second bond dissociation energies, BDE[Pa+-O] and BDE[OPa+-O], are approximately 800 kJ mol(-1). The relatively facile fluorination of Pa+ to PaF4+ by SF6 is consistent with the high stability of the pentavalent oxidation state of Pa. Reactions with ethene, propene, 1-butene, and iso-butene revealed that Pa+ is a very reactive metal ion. In analogy with U+ chemistry, ethene was trimerized by Pa+ to give PaC6H6+. Reactions of Pa+ with larger alkenes resulted in secondary and tertiary products not observed for U+ or Np+. The bare protactinium ion is significantly more reactive with organic substrates than are heavier actinicle ions. The greatest difference between Pa and heavier actinide congeners was the exceptional dehydrogenation activity of PaO+ with alkenes; UO+ and NpO+ were comparatively inert. The striking reactivity of PaO+ is attributed to the distinctive electronic structure at the metal center in this oxide, which is considered to reflect the greater availability of the 5f electrons for participation in bonding, either directly or by promotion/hybridization with higher-energy valence orbitals.