Transmetalation is the stoichiometric replacement of the metals in a polymetallic target with other metals from reagents called transmetalators. The family of heteropolymetallic complexes (mu4-O)L4Cu4-xMxCl6 (M = Ni and x = 0-4 in I-V, respectively) was obtained by transmetalating the parent (x = 0) with x mol of Ni(NS)2, where L is N,N-diethylnicotinamide and NS is S-methyl isopropylidenehydrazinecarbodithioate. These and other neutral polymetallic complexes VI-XI containing different combinations of Co, Ni, Cu, and Zn were evaluated by fast atom bombardment mass spectrometry (FAB/MS). Ionization of I-XI by FAB/MS produced no molecular ions under the analysis conditions employed, but the production of characteristic fragments as well as the formation of several metal-ligand complexes was observed. No heteropolymetallic fragments were observed in this study. The spectral patterns of the mixed metal complexes II-IV and VI-X indicate that metal exchange occurs in the matrix as a result of exposure to the FAB beam. (Mu4-O)L4CuNi3Cl6 produces a fragmentation pattern indistinguishable from that of (mu4-O)L4Cu4Cl6 in the mass range from m/z 650 and above, yet produces ions at m/z 419 and 449 (CuL2+ and [NiCl]L2+, respectively) in ratios expected from the stoichiometric metal content of the original complex. It appears that the m/z 419 and 449 peaks do not result from an ionically induced fragmentation of the parent complex but rather from decomposition of the complex and recombination of neutral ligand and metal species in the nitrobenzyl alcohol matrix. Isotopic abundance calculations were used to determine fragmentation pathways and fragment compositions. A mixture of (mu4-O)L4Cu4Cl6(I) and (mu4-O)L4Ni4Cl6(V) in 3-nitrobenzyl alcohol produced a spectrum whose major fragments were remarkably different from those of the heteropolymetallic copper/nickel complexes (II-IV).