Abstract: The positive Ion FAB-MS of Cluster complex Co3Fe (mp)4 (Hmp) (PBu)3 (H2mp＝2－mercaptophenol, PBu＝tri－n－butylph0sphine ) are reported. In the light of thespectra and the molecular structural date the major fragmentation pathways are proposed.The peak is represented by m/z 1463 which corresponds to the cationCo3Fe (mp)4(Hmp) (PBu)3·3H orCo4Fe (mp)4 (Hmp) (PBu)3. The latter ion may be ratherrational and it is related to the metal exchange reaction of Co/Fe occurring in the matrixas a result of exposure to the FAB beam. The Competitive cleavage of Hmp and PBufrom the molecular ion occurred later and the cations FeCo3 (mp)4 (PBu)3＋(m/z1335) and FeCo3 (mp)4(Hmp) (PBu)2＋(m/z 1258 ) were observed, respectively. Thelatter ion then cleaved by either Hmp or Fe (Hmp) and PBu to give Co3Fe (mp)4(PBu)2 (m/z 1133 )or Co3 (mp)4 (PBu)＋(m/z 875 ), respectively.