The reaction of [8,8-(PPh3)2-nido-8,7-RhSB9H10] (1) with PR3 in a 1 : 2 ratio affords mixtures that contain the mono-substituted bis-PR3-ligated rhodathiaboranes [8,8-(PPh3)(L)-nido-8,7-RhSB9H10] [L = PMe2Ph (5), PMe3 (6)] and the corresponding tris-PR3-ligated compounds [8,8,8-(L)3-nido-8,7-RhSB9H10] [L = PMe2Ph (7), PMe3 (8)]. These latter species are more conveniently prepared from the reaction of 1 with three equivalents of the monodentate phosphines, PMe2Ph and PMe3. Reaction between 1 and PMePh2 in a 1 : 2 ratio yields the disubstituted rhodathiaborane [8,8-(PMePh2)2-nido-8,7-RhSB9H10] (4), whereas the use of three equivalents of phosphine leads to the formation of B-ligated eleven-vertex [8,8,8-(PMePh2)2(H)-nido-8,7-RhSB9H9-9-(PMePh2)] (9). Compounds 4–9 have been characterized by NMR spectroscopy, and the structures of 8 and 9 confirmed by X-ray diffraction analyses. The characterization of the cluster compounds has been aided by the use of DFT calculations on some of the species. Variable-temperature NMR studies have demonstrated a lability of the PMePh2 ligands in compounds 4 and 9, providing mechanistic insights about the ligand substitutional chemistry in these eleven-vertex rhodathiaboranes.