Positive and negative ion FAB-MS (fast atom bombardment mass spectrometry) has been used to help characterize large polyoxoanions with molecular weights of up to 6300 mass units, specifically [BU4N]9P2W15Nb3O62 and the important polyoxoanion-supported iridium organometallic complex [Bu(4)N]Na-5(3)[(1,5-COD)Ir.P2W15Nb3O62]. A detailed assignment of more than 50 envelopes has been performed, including the comparison of experimental vs calculated molecular ion distributions for key fragments. Two main pathways account for the most common fragmentations in both positive and negative ion FAB-MS, the loss of WO3 and the loss of O (and/or H2O, the latter generally following cation exchange between H+ from the matrix and Bu(4)N(+) or Na+). The excellent agreement between the experimental and calculated isotopic ion distributions provides reliable molecular formulas, even for fragments differing by only a few mass units. A higher degree of fragmentation is observed in the negative ion mode; this provides interesting structural and mechanistic results, notably the formation of a "P2W10Nb2O424-" fragment analogous to the "P2W12" polyoxoanion described in the literature. Overall, the most important finding is that most of the peaks in such complicated mass spectra can be readily assigned in terms of simple mechanisms and now-established polyoxoanion-fragmentation chemistry.