Cationic, phosphine-ligated Pt-Au cluster compounds, [Pt(AuPPh(3))(8)](NO3)(2) (1) and [(PPh(3))Pt(AuPPh(3))(6)](NO3)(2) (2), have been immobilized on silica and alumina supports. Characterization of the supported clusters by P-31 MAS NMR, IR, and W-visible spectroscopy and study by chemisorption and temperature-programmed desorption of CO show that the clusters are immobilized intact without measurable fragmentation or irreversible transformation. This is confirmed by quantitative desorption of the clusters. The reactivity of the supported clusters with CO and H-2 was found to be similar to the known reactivities in solution and molecular solid phases. The cluster [(PPh(3))-Pt(H)(AuPPh(3))(7)](NO3)(2) (3) was also immobilized on silica and found to be intact by MAS P-31 NMR and desorption experiments. The turnover rate (TOR) for H-2-D-2 equilibration was determined for these supported clusters at room temperature. The clusters 1 and 2 were good catalysts on silica and alumina supports after treatment at 135 degrees C under vacuum (TOR = 10-20 s(-1)), but showed significant activation by treatment at 110 degrees C under H-2 (TOR = 85-220 s(-1)). Cluster 3/SiO2 gave a similar rate (TOR = 170 s(-1)) with no thermal activation. All samples subjected to the thermolysis conditions given above were found to be intact by P-31 MAS NMR and could be quantitatively desorbed. The high activities of the supported Pt-Au clusters are believed to result from support-promoted, partial PPh(3) dissociation. Desorption experiments show that this dissociation is reversible.