Bimetallic Pt-Au catalysts supported on ZnO/Al2O3 were prepared by the impregnation (IMP) and incipient wetness impregnation (IW-IMP) methods in air or H-2 and compared with a monometallic Pt/ZnO/Al2O3 catalyst. The catalysts were characterized by ultraviolet-visible spectroscopy (UV-vis), X-ray diffraction (XRD), CO chemisorption, temperature-programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS) and scanning transmission electron microscopy (STEM) in conjunction with energy dispersive spectroscopy (EDS). The catalytic activity for the complete oxidation of toluene was measured using a flow reactor under atmospheric pressure. The relationship between the particle size and catalytic activity for toluene over bimetallic Pt-Au catalysts is discussed. In the results, generally, the Pt particles prepared by IW-IMP in H-2 were larger than those prepared by IMP in air. On the other hand, the Au particles prepared by IW-IMP in H-2 were smaller than those prepared by IMP in air, namely when using IW-IMP in H-2 the Au particle size was decreased and the Pt particle size was increased. Also, the particle sizes of Pt and Au increased with increasing calcination temperature. The STEM and XRD results show that Pt and Au were simultaneously deposited as metallic particles on the ZnO/Al2O3 without forming an alloy with the Pt and Au. From the TPR results, it was found that the nanosized Au particles might promote the reduction of the surface oxygen and that the complete oxidation of toluene shows higher activity at lower temperature over the bimetallic Pt-Au catalysts as compared to the Pt/ZnO/Al2O3 catalyst. The bimetallic Pt-Au catalysts prepared by IW-IMP in H-2 calcined at 400 degrees C showed higher activity for complete toluene oxidation. (C) 2009 Elsevier B.V. All rights reserved.