Direct epoxidation of propylene by oxygen was investigated over Ag-based catalysts containing an added 3d transition-metal species (Mn, Fe, Co, or Ni). The oxidation of propylene occurred at lower temperatures over Ag-based catalysts containing small amounts of added metal species than over a Ag single catalyst. The yield of propylene oxide (PO) depended strongly on the type of metal: the addition of Ni afforded the highest yield of PO at similar values for the conversion of propylene (4-6%). The highest PO selectivity, four times that obtained with the Ag single catalyst, was obtained at 33 mol% Ni. X-ray diffraction analysis and transmission electron microscopy results showed that the dispersion of Ag particles in the Ag-Ni catalysts increased with increasing Ni content, which suggests that Ni atoms controlled the Ag particle size by suppressing the sintering of Ag particles. The turnover frequency for propylene epoxidation was highest at a Ag particle size of approximately 80 nm. (c) 2005 Elsevier B.V. All rights reserved.