Three new compounds, PtOs3(CO)(12)(PBu3t) (10), Pt2Os3(CO)(12)(PBu3t)(2) (11), and Pt3Os3(CO)(12)(PBu3t)(3) (12), have been obtained from the reaction of Pt(PBu3t)(2) with Os-3(CO)(12) (9). The products were formed by the sequential addition of 1-3 Pt(PBu3t) groups to the three Os-Os bonds of the metal cluster of Os-3(CO)(12). In solution, compounds 10-12 interconvert among themselves by intermolecular exchange of the Pt(PBu3t) groups. When 11 is treated with PPh3, the mono- and bis(PPh3) derivatives of 9, Os-3(CO)(11)(PPh3) and Os-3(CO)(10)(PPh3)(2), were obtained by elimination of the Pt(PBu3t) groups together with one and two CO ligands, respectively. When heated, compound 11 was transformed into the new compound Pt2Os3(CO)(10)(PBu3t)((PBu2CMe2CH2)-C-t)(mu-H) (13) by the loss of two CO ligands and a metalation of one of the methyl groups of one of the PBu3t ligands. Compounds 10-13 have been characterized by single-crystal X-ray diffraction analyses.