Highly dispersed gamma-Al2O3-supported PtRu clusters were prepared by decarbonylation of molecularly adsorbed [Pt2Ru4(CO)(18)] in He or H-2 at temperatures in the range of 300-400 degreesC. Infrared JR) and extended X-ray absorption fine structure (EXAFS) spectroscopies were used to characterize the supported species before and after removal of the CO ligands. The IR and EXAFS spectra show that most of the [Pt2Ru4(CO)(18)] interacted weakly through its CO ligands with the surface hydroxyl groups of gamma-Al2O3 and could be extracted into CH2Cl2, but a small fraction of the [Pt2Ru4(CO)(18)] interacted strongly with the support and could not be extracted. The strongly bonded species, characterized by a v(co) band at 2063 cm(-1), formed as a result of nucleophilic attack of some support hydroxyl groups on carbonyl ligands, leading to the removal of these ligands as CO2 and formation of covalent bonds between the clusters and the oxygen atoms of the support. The EXAFS data show that after decarbonylation in He or H-2 at 300-400 degreesC, the Pt-Ru cluster frame was changed relative to that of the supported [Pt2Ru4(CO)(18)]. The average Pt-Pt bond distance increased from 2.66 to 2.69 Angstrom, and the Ru-Ru distance decreased from 2.83 to 2.64 Angstrom. The corresponding Pt-Pt and Ru-Ru coordination numbers were found to be 2.0 and 4.0, respectively, indicating that slight agglomeration of the metal took place upon decarbonylation. The EXAFS data show that the Pt-Ru interactions were largely maintained after decarbonylation, so that highly dispersed Pt-Ru structures were present on the support. From the Pt-Pt and Ru-Ru coordination numbers it follows that decarbonylated clusters incorporate, on average, less than three and six Pt and Ru atoms, respectively, being the smallest such supported bimetallic clusters of platinum-group metals yet reported.