Pt promoted and unpromoted Pd/TS-1 catalysts were prepared by impregnation with [Pd(NH3)(4)](NO3)(2) and [Pt(NH3)(4)]Cl-2, characterized by TPR, TEM, and ESCA and tested as catalysts for the synthesis of propylene oxide (PO) from propylene, oxygen, and hydrogen. The effect of various reduction methods and Pt loading on the catalytic performance and physical characteristics of the catalysts have been studied. The autoreduction by the amine ligands under a flow of N-2 at 150 degrees C leads to maximum PO selectivities and yields, whereas catalysts that were hydrogen reduced or calcined prior to reduction were less effective. Results obtained by ESCA and TEM show that PO formation was favoured by small Pd clusters and a high fraction of Pd(II) species. Pd agglomeration was detected on the outer surface of TS-1 crystals and was especially pronounced for calcined and reduced catalysts with cluster sizes up to 70 mm. Maximum PO yield (11.7%) with a selectivity of 46% were obtained over a TS-1 catalysts loaded with 1 wt% Pd and 0.02 wt% Pt and autoreduced under N-2 at 150 degrees C. The increase in PO yield by adding minor amounts of Pt to a Pd/TS-1 catalyst correlates with a sharp increase in the fraction of Pd(II) species. Further addition of Pt changed the form and size of Pd clusters without further increasing the fraction of Pd(II) species, thus leading to decreasing PO yields and selectivities.