Swapping of an oxygen atom of water with that of a pentavalent actinide dioxide cation, AnO(2)(+) also called an "actinyl", requires activation of an An-O bond. It was previously found that such oxo exchange in the gas phase occurs for the first two actinyls, PaO2+ and UO2+, but not the next two, NpO2+ and PuO2+. The An-O bond dissociation energies (BDEs) decrease from PaO2+ to PuO2+, such that the observation of a parallel decrease in the An-O bond reactivity is intriguing. To elucidate oxo exchange, we here extend experimental studies to AmO2+, americyl(V), and CmO2+, curyl(V), which were produced in remarkable abundance by electrospray ionization of Am3+ and Cm3+ solutions. Like other AnO(2)(+), americyl(V) and curyl(V) adsorb up to four H2O molecules to form tetrahydrates AnO(2)(H2O)(4)(+) with the actinide hexacoordinated by oxygen atoms. It was found that AmO2+ does not oxo-exchange, whereas CmO2+ does, establishing a "turn" to increasing the reactivity from americyl to curyl, which validates computational predictions. Because oxo exchange occurs via conversion of an actinyl(V) hydrate, AnO(2)(H2O)(+), to an actinide(V) hydroxide, AnO(OH)(2)(+), it reflects the propensity for actinyl(V) hydrolysis: PaO2+ hydrolyzes and oxo-exchanges most easily, despite the fact that it has the highest BDE of all AnO(2)(+). A reexamination of the computational results for actinyl(V) oxo exchange reveals distinctive properties and chemistry of curyl(V) species, particularly CmO(OH)(2)(+).