This study shows how different morphologies of silver nanoparticles affect the selective oxidation of styrene in the gas phase using oxygen as oxidant. Silver nanoparticles (nanowires and nanopolyhedra), prepared using the polyol process, were supported on alpha-Al2O3. For comparison, a conventional catalyst obtained by wet impregnation was also prepared. Phenylacetaldehyde (Phe) and styrene oxide (SO) were the main products for nanoparticles catalysts. The promotion effect on the catalytic activity of potassium and cesium on the silver nanowires catalysts was also studied. At 573 K, the styrene conversion and selectivity to styrene oxide with the silver nanowires catalyst were 57.6 and 42.5%, respectively. Silver nanopolyhedra catalyst showed 57.5% conversion and 30.8% selectivity to styrene oxide. The promotion by cesium played an important role in improving the epoxidation of styrene. The samples were structurally characterized using X-ray diffraction (XRD), ultraviolet-visible spectroscopy (UV-vis), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). X-ray photoelectron spectroscopy (XPS) and temperature programmed reduction (TPR) were applied to characterize the oxygen species detected (O-beta, O-gamma) on the silver surface. (c) 2005 Elsevier B.V. All rights reserved.