Platinum clusters smaller than 1.5 nm on gamma-alumina support are stable against thermal sintering in hydrogen at temperatures as high as 500 degrees C. The small Pt clusters exhibited a high degree of electron deficiency as determined by X-ray photoelectron spectroscopy (XPS) and CO infrared spectroscopy. The origin of the electron deficiency is proposed to be due to (i) the strong interaction of small Pt particles with the positively charged Lewis acid sites, on the alumina surface, and (ii) the intrinsic electronic character of small Pt clusters. The stability of small Pt clusters against thermal sintering can also be ascribed to the anchoring effect of Pt by the acid sites on the surface. Upon CO adsorption on the small Pt clusters, IR absorption bands indicated the formation of Pt carbonyl clusters. The linearly adsorbed CO on Pt is characteristic of small Pt clusters. A band at 2112 cm(-1) suggests the presence of charged Pt atoms. Bridged CO infrared bands, 1825 and 1661 cm(-1), which are absent from bulk Pt, are also evidence for the formation of small Pt carbonyl clusters. It was discovered through this work that the small Pt clusters coalesce readily upon H-2 reduction at 320 degrees C when the catalyst is pretreated with a solution of a chloride salt such as NH4Cl. The agglomeration of small Pt clusters in the presence of a chloride salt under a reducing environment is ascribed to the deanchoring effect of the chloride, which weakens the interaction of Pt with the anchoring Lewis acid sites. As temperature increases, the formation of mobile platinum chloride complexes that leads to Pt particle agglomeration is proposed. The Pt particle sizes after the reductive agglomeration are distributed within a narrow range of 5-8 nm. The electronic character of the large Pt particles is identical to that of bulk Pt metal. It was further demonstrated that Pt cluster agglomeration is independent of the heating rate and the length of heating period, indicating that the predominant mechanism of Pt particle agglomeration is due to rapid coalescence of chlorinated primary small Pt particles under an H-2 atmosphere.