The study of gas-phase organometallic chemistry has been extended to the first two transuranium elements, Np and Pu. Product abundances were determined for reactions of the alkenes, L = ethene, cis-2-butene, cyclohexene, and 1,5-cyclooctadiene (COD), with Np+, NpO+, Pu+, and PuO+, and comparison was made with results for Th+, U+, ThO+, and UO+. A key finding was that U+ and Np+ were comparably effective at dehydrogenation of ethene and 2-butene whereas Pu+ was less effective, consistent with a conventional mechanism of dehydrogenation via oxidative insertion of a prepared "divalent" M+ into a C-H bond (i.e., C-An(+)-H, where An = actinide). The reduced reactivity of Pu+ indicates that its 5f electrons do not participate in C-H bond activation. The smaller discrepancy between U+ and Np+ compared with Pu+ which was found using cyclohexene and COD as reactants can be attributed to their greater polarizabilities and allylic C-H bonds. The linear alkenes were essentially inert toward all of the AnO(+), but cyclohexene was dehydrogenated by UO+ and ThO+ and COD was dehydrogenated by all four AnO(+). With COD, single H-2 loss was dominant for NpO+ and PuO+, while double dehydrogenation was the primary pathway for UO+ and ThO+; it appears that C-H activation occurred by different mechanisms and that the 5f electrons of UO+ may facilitate C-H activation whereas the 5f electrons of NpO+ and PuO+ are inert. The relatively high reactivity of ThO+ accords with the classification of Th as a pseudo-d-block transition element.