Gold has long been thought to be chemically inert, however, it has recently been proven that its catalytic performance is dramatically tunable by control of the particle size and by careful selection of the support metal oxide. A typical example is the selective oxidation of propylene in a gas containing oxygen and hydrogen. When gold is deposited on TiO2 by a deposition-precipitation technique as hemispherical particles with diameters smaller than 4.0 nm it produces propylene oxide with selectivities higher than 90% and conversions of 1-2% at temperatures of 303-393 K, The oxidation of hydrogen to form water is depressed by propylene, whereas propylene oxidation is not only enhanced but also restricted to partial oxidation by hydrogen. The depression of hydrogen combustion by the presence of propylene and a new peak due to gold deposition in TPD spectra have indicated that propylene is adsorbed on the surfaces spectra have indicated that propylene is adsorbed on the surfaces of both gold particles and the TiO2 support. The reaction rate is almost independent on the concentration of propylene and increases linearly with increasing concentrations of O-2 and H-2. The above results suggest that propylene adsorbed on a gold surface may react with oxygen species formed at the perimeter interface between the gold particles and the TiO2 support through the reaction of oxygen with hydrogen. The effect of gold loading is surprising in that the reaction product switches from propylene oxide to propane when gold loading is decreased to 0.1 wt%. Careful TEM observation indicates that gold particles larger than 2.0 nn in diameter produce propylene oxide, whereas smaller gold particles produce propane.