The reaction of the [Pt-3n(CO)(6n)](2-) (n = 2-6) Chini's clusters with increasing amounts of PPh3 has been investigated in detail by combined FT-IR, P-31{H-1} NMR, and electrospray ionization-mass spectrometry (ESI-MS) studies, showing that up to three CO ligands are gradually substituted by PPh3, resulting in isonuclear phosphine-substituted anionic clusters of general formula [Pt-3n(CO)(6n-x)(PPh3)](2-) (n = 2-6; x = 1-3). Further addition of PPh3 results in the elimination of the neutral Pt-3(CO)(3)(PPh3)(3) species and formation of lower nuclearity anionic clusters. [Pt-12(CO)(22)(PPh3)(2)](2-) and [Pt-9(CO)(16)(PPh3)(2)](2-) have been structurally characterized, and they maintain the trigonal prismatic structures of the parent homoleptic clusters, with the two PPh3 ligands bonded to different external Pt-3-triangles in relative cis-position. Conversely, the crystal structure of [Pt-6(CO)(10)(PPh3)(2)](2-) shows that its metal cage is transformed from trigonal prismatic to trigonal antiprismatic after CO/PPh3 exchange.