The first gas-phase ion chemistry studies of the transuranium actinides Np and Pit by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FTICR-MS) have been performed. Reactions of An(+) and AnO(+) (An = Th, U, Np, and Pu) with N2O, C2H4O (ethylene oxide), H2O, O-2, CO2, NO, and CH2O have been studied, with a focus on the oxidation of transuranium ions. All the An(+) ions formed AnO(+) with all the oxidants studied. with the exception of Pu+ with CH2O, in accord with the known bond dissociation energies, BDE(An(+)-O). The reaction efficiencies appear to correlate with the magnitude of the promotion energies from the ground states to reactive-state configurations of the An(+) ions. Only N2O and C2H4O oxidized all AnO(+) to AnO(2)(+); CO2 and NO oxidized only UO+; H2O and O-2 oxidized UO+, NpO+, and PuO+; and CH2O was unreactive with all the AnO(+) ions. The observed formation of PuO2+ from the oxidants weaker than N2O is in conflict with the literature values for BDE(OPu+-O), which we conclude are significantly too low. Charge-transfer "bracketing" experiments to determine the ionization energy of PuO2 were performed, from which a value of 7.03+/-0.12 eV was obtained. This IE(PuO2) is 2-3 eV lower than the literature values, but in accord with our observations for the oxidation thermodynamics.