Samples of Pt on gamma-Al2O3 were prepared by reduction in H-2 at 400 degrees C of [PtCl2(PhCN)(2)], adsorbed intact from n-pentane solution onto gamma-Al2O3 powder that had been partially dehydroxylated under vacuum at 400 degrees C. The Pt dispersion was determined to be about 0.58 by standard methods, namely, chemisorption of hydrogen, of oxygen, and of CO, titration of chemisorbed oxygen with hydrogen, and extended X-ray absorption fine structure (EXAFS) spectroscopy. The supported Pt particles were resistant to sintering in the presence of O-2 at temperatures up to 400 degrees C, as shown by chemisorption and EXAFS data, but the latter indicated that a combination of O-2 and H-2 treatments at 200-400 degrees C led to changes in particle morphology. The samples were investigated by temperature-programmed desorption (TPD) of preadsorbed hydrogen; a TPD peak centered at 120 degrees C represents hydrogen adsorbed on Pt and confirms the value of 0.58 for the dispersion. TPD of preadsorbed H-2 gave evidence of a second desorption peak, at 580 degrees C, which was always accompanied by desorption of water at the same temperature; its presence was found to depend chiefly on the sample reduction temperature and evacuation time (it was absent when there was no Pt on the gamma-Al2O3) The desorption of H-2 or D-2 that had been adsorbed on Pt/gamma-Al2O3 reduced with D-2 or H-2, respectively, shows that hydrogen species desorbed at about 580 degrees C originate from water (hydroxyl) species as a result of Pt-catalyzed decomposition.