The reactions of [Pt-3n(CO)(6n)](2-) (n = 2-5) homoleptic Chini-type clusters with increasing amounts of 1,3,5-triaza-7-phosphaadamantane (PTA) result in the stepwise substitution of one terminal CO ligand per Pt-3 triangular unit up to the formation of [Pt-3n(CO)(5n)(PTA)(n)](2-) (n = 2-5). Competition between the nonredox substitution with retention of the nuclearity and the redox fragmentation to afford lower nuclearity heteroleptic Chini-type clusters is observed as a function of the amount of PTA and the nuclearity of the starting cluster. Because of this, [Pt-12(CO)(20)(PTA)(4)](2-) and [Pt-15(CO)(25)(PTA)(5)](2-) are more conveniently obtained via the oxidation of [Pt-9(CO)(15)(PTA)(3)](2-). All the new species were spectroscopically characterized, and the structures of [Pt-12(CO)(20)(PTA)(4)](2-) and [Pt-15(CO)(25)(PTA)(5)](2-) were determined by single-crystal X-ray diffraction. These clusters may be viewed as heteroleptic Chini-type clusters composed of stacks of four and five Pt-3(-CO)(3)(CO)(2)(PTA) units, respectively. The solubility in water of [Pt-12(CO)(20)(PTA)(4)](2-) and [Pt-15(CO)(25)(PTA)(5)](2-) has been determined and their cytotoxicity towards human ovarian (A2780) cancer cells and their cisplatin-resistant strain (A2780cisR) has been evaluated.