© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Actinyl chemistry is extended beyond Cm to BkO2+and CfO2+through transfer of an O atom from NO2to BkO+or CfO+, establishing a surprisingly high lower limit of 73 kcal mol−1for the dissociation energies, D[O-(BkO+)] and D[O-(CfO+)]. CCSD(T) computations are in accord with the observed reactions, and characterize the newly observed dioxide ions as linear pentavalent actinyls; these being the first Bk and Cf species with oxidation states above IV. Computations of actinide dioxide cations AnO2+for An=Pa to Lr reveal an unexpected minimum for D[O-(CmO+)]. For CmO2+, and AnO2+beyond EsO2+, the most stable structure has side-on bonded η2-(O2), as AnIIIperoxides for An=Cm and Lr, and as AnIIsuperoxides for An=Fm, Md, and No. It is predicted that the most stable structure of EsO2+is linear [O=EsV=O]+, einsteinyl, and that FmO2+and MdO2+, like CmO2+, also have actinyl(V) structures as local energy minima. The results expand actinide oxidation state chemistry, the realm of the distinctive actinyl moiety, and the non-periodic character towards the end of the periodic table.