Noble metals such as Au, Ag and Cu supported over semiconducting ZnO are well-known heterogeneous oxidation catalysts. All of them have been utilized for the oxidation of diesel soot with varied success. However, Au-supported ZnO is seen to be superior among them. Here we present a comparative study of all these three catalysts for diesel soot oxidation, to explain why Au/ZnO is the best among them, demonstrating the contribution of electronic states of metals in composite catalysts. The electronic states of Cu, Ag, and Au determined by X-ray photoelectron spectroscopy (XPS) on 1wt%Cu/ZnO, 1wt%Ag/ZnO, and 1wt%Au/ZnO catalysts were correlated with their diesel soot oxidation activities. Although all three catalysts present reasonable diesel soot oxidation activities at relatively low temperature, while 1%Cu/ZnO and 1%Ag/ZnO oxidize only about 60% of the deposited diesel soot around 250°C, 1%Au/ZnO oxidizes 100% of the deposited diesel soot, at temperature as low as 230°C. The activity of the catalysts is attributed to the formation of stable M0–M?+ bifunctional catalytic sites at the metal-ZnO interface, which enhances the contact efficiency of solid diesel soot on M?+ and generates superoxide species on M0 moieties. The stability of the bifunctional M0–M?+ sites is controlled by the electronic interactions between the metal (M) and n-type semiconductor ZnO at their interface. Very high activity of 1%Au/ZnO is attributed to the presence of Au3+ at the catalyst surface which generates a stronger coulombic force with diesel soot electrons. We demonstrate a direct relation between the diesel soot oxidation activity of these three metals and their electronic states at the catalyst surface.