Reported here is a comparison of electron transfer dissociation (ETD) and collision-induced dissociation (CID) of solvent-coordinated dipositive uranyl and plutonyl ions generated by electrospray ionization. Fundamental differences between the ETD and CID processes are apparent, as are differences between the intrinsic chemistries of uranyl and plutonyl. Reduction of both charge and oxidation state, which is inherent in ETD activation of [AnVIO2(CH3COCH3)4]2+, [AnVIO2(CH3CN)4]2, [UVIO2(CH3COCH3)5]2+ and [UVIO2(CH3CN)5]2+ (An=U or Pu), is accompanied by ligand loss. Resulting low-coordinate uranyl(V) complexes add O2, whereas plutonyl(V) complexes do not. In contrast, CID of the same complexes generates predominantly doubly-charged products through loss of coordinating ligands. Singly-charged CID products of [UVIO2(CH3COCH3)4,5]2+, [UVIO2(CH3CN)4,5]2+ and [PuVIO2(CH3CN)4]2+ retain the hexavalent metal oxidation state with the addition of hydroxide or acetone enolate anion l! igands. However, CID of [PuVIO2(CH3COCH3)4]2+ generates monopositive plutonyl(V) complexes, reflecting relatively more facile reduction of PuVI to PuV