A series of Pt-Au and related phosphine-stabilized cluster compounds have been shown to be excellent catalysts for H-2-D2 equilibration (H-2 + D2 = 2HD) under homogeneous and heterogeneous conditions. H-2-D2 equilibration is a model reaction for catalytic H-2 activation. Turnover rates for HD production (ca. 5 s-1 for the best catalyst) were comparable to those of activated Pt surfaces. Kinetic data measured under homogeneous conditions(nitrobenzene solvent, 30-degrees-C, 1 atm) showed the following trends in rate : (i) Faster rates were observed as the Au/Pt ratio increased. (ii) Pt-Au clusters gave faster rates than Pd-Au clusters; Au clusters were inactive. (iii) Sixteen-electron clusters were more active than 18-electron clusters; 18-electron hydrido clusters were also very active. These trends suggested that the M-Au bonds may function as active sites for H-2 activation (mu2-H and mu3-H bonding occurs in known hydrido clusters) with Pt-Au bonds being more active than Pd-Au bonds. The mechanism for the homogeneous, catalytic H2-D2 equilibration was studied by spectroscopic and kinetic measurements. For the 16-electron cluster [Pt(AuPPh3)8](NO3)2=M, it is thought to involve the following steps : (1) M + H-2=M(H)2; (2)M(H)2=M*(H)2+PPh3; (3)M’(H)2+D2=M*(H)2(D)2. Step 1 is directly observable by NMR spectroscopy, while steps 2 and 3 were implicated by kinetic studies as a function of H-2, D2, and metal cluster concentration and by the effect of added PPh3. Step 3, the formation of a 20-electron, tetra(hydrido, deuterio) species, is rate limiting and its reverse leads to HD production. The mechanism with the hydrido, 18-electron cluster [Pt(H)(PPh3)(AnPPh3)7](NO3)2 is also thought to involve PPh3 dissociation followed by the rate-limiting addition of D2 giving a 20-electron, trihydrido species or activated complex. The 20-electron species have not been directly observed. These phosphine-stabilized clusters have deep and narrow hydrophobic channels into the metal core which prevents binding of many larger substrates thus protecting the core from poisoning. This helps explain the high turnover rates for HD production observed with molecular solids of these clusters. This study has shown that phosphine-stabilized, mixed-metal cluster compounds are useful as models for polymetallic surface catalysts and that structure-reactivity relations have relevance to this important area in catalysis.