Samples prepared by the reaction of [Pt(C3H5)(2)] With MgO powder were treated in H-2 at temperatures in the range 25-150 degrees C and characterized by X-ray absorption spectroscopy and infrared spectroscopy. As the temperature of treatment in H-2 was increased, the infrared spectra indicated the disappearance of bands attributed to the allyl ligands, and the sample became gray, the color characteristic of a supported metal. Adsorption of CO was characterized by a single infrared peak in the nu(CO) region at 2070 cm(-1), consistent with that observed for CO chemisorbed on metallic Pt, The Fourier transforms of the EXAFS functions measured near the Pt L(III) absorption edge for each sample confirm the presence of metallic Pt, showing peaks at approximately 2.7, 3.8, 4.7, and 5.6 Angstrom, consistent with the first, second, third, and fourth nearest neighbors in bulk fee Pt. The EXAFS results indicate that the particle size increased from approximately 8 Angstrom after treatment in H-2 at room temperature, to about 9 Angstrom after a similar treatment at 50 degrees C, to about 10 Angstrom after treatment at 150 degrees C. The metal-support interface was characterized by two metal-support oxygen contributions, one at 2.18 Angstrom and one in the range 2.7-2.9 Angstrom. The total Pt-O coordination number decreased monotonically as the treatment temperature in H-2 increased. This result confirms that the fraction of Pt atoms in contact with the support decreased as the particle size increased.