Electrospray ionization (ESI) of aqueous cobalt(II) and nickel-(II) nitrate solutions inter alia affords the solvated, mono-and oligonuclear nitrato complexes [M m + (M = Co, Ni; m = 1-5; n = 1-4). The collision-induced dissociation spectra of the mass-selected ions imply that these ions correspond to genuine hydrated metal(II) nitrato complexes in that either cluster degradation through expulsion of neutral M(NO 3 ) 2 or sequential loss of water ligands take place. In the case of the lowest member of the series (m, n = 1), however, loss of water competes with homolytic cleavage of the N-O bond, which leads to the formation of [M,O 2 ,H 2 ] + cat-