Electrospray ionization-mass spectrometry (ESI-MS) is used to investigate the complexation of nickel(II) by N,N-dimethylformamide (DMF) in different water/DMF mixtures. The types of solvated cations observed are independent of the solvent composition and are part of the series [Ni(DMF)(n)](2+) with n = 2-6 and [Ni(DMF)(n)X](+) with n = 1-4 (where X = Cl, Br, NO3). The number of ligands, n, and thus the extent of solvation depends on the cone voltage (U-C) of the ESI source. At low U-C multiply ligated ions prevail, whereas ligation drops with rising U-C and further increase of U-C causes dissociation of the solvent molecules as well as reduction of Ni(II) to Ni(I). The collision-induced dissociation (CID) spectra of the multiply ligated ions [Ni(DMF)(n)](2+) with n greater than or equal to 3 and [Ni(DMF)(n)X](+) with n greater than or equal to 2 show only losses of neutral DMF ligands. Quite different is the behavior of the bisligated ions [Ni(DMF)(2)](2+) and [Ni(DMF)X](+). In the CID spectrum of [Ni(DMF)(2)](2+), electron transfer from DMF to the metal leads to the reduced complex [Ni(DMF)](+) concomitant with ionized DMF+. Charge-stripping spectra of mass-selected monocations [Ni(DMF)(n)](+) (n = 1, 2) confirm that the dication [Ni(DMF)(2)](2+) is stable, whereas monoligated [Ni(DMF)(2)](2+) appears to be intrinsically unstable toward Coulomb explosion. In the CID spectra of [Ni(DMF)Cl](+) and [Ni(DMF)Br](+), bond activation of the solvent takes place, and in the case of [Ni(DMF)NO3](+) the formation of a cationic species is observed which formally corresponds to the solvated metal-oxide cation [(DMF)NiO](+).