MgO-supported platinum and platinum-molybdenum catalysts were prepared from organometallic precursors and characterized structurally to determine how the nature of the bimetallic precursors and the treatment conditions affected the interaction between the two metals. Samples were prepared from [PtCl2(PhCN)(2)], [PtCl2(PhCN)(2)] + [Mo(CO)6], and {Pt[Mo(CO)(3)(C5H5)](2)(PhCN)(2)} and characterized by infrared and extended X-ray absorption fine structure (EXAFS) spectroscopies, transmission electron microscopy, and chemisorption of H-2, CO, and O-2. The samples were treated in H-2 at 400 degrees C prior to most of the characterizations. Incorporation of Mo reduced the chemisorption of CO and of H-2. EXAFS spectra measured at the Pt L(III) edge and at the Mo K edge showed substantial Pt-Mo contributions with a Pt-Mo coordination number of about 2 and an average distance of 2.63 Angstrom for the sample prepared from {Pt[Mo(CO)(3)(C5H5)](2)(PhCN)(2)}. In contrast, no significant Pt-Mo contribution was observed for the sample prepared from [PtCl2(PhCN)(2)] + [Mo(CO)6]. Electron micrographs and EXAFS results show that interaction between Pt and Mo ions in the former sample helped to maintain the platinum in a highly dispersed form, with supported platinum clusters being smaller than about 10 Angstrom.