The effects of palladium precursors (PdCl2, (NH4)(2)PdCl4, Pd(NH3)(2)Cl-2, Pd(NO3)(2) and Pd(CH3COO)(2)) on the catalytic properties in the selective oxidation of ethylene to acetic acid have been investigated for 1.0wt% Pd-30wt% H4SiW12O40/SiO2. The structures of the catalysts were characterized using X-ray diffraction, N-2 adsorption, H-2-pulse chemical adsorption, infrared spectrometry of the adsorbed pyridine, H-2 temperature-programmed reduction and X-ray photoelectron spectroscopy. The present study demonstrates that the different palladium precursors can lead to the significant changes in the dispersion of palladium. It is found that Pd dispersion decreases as follows: PdCl2>(NH4)(2)PdCl4>Pd(NO3)(2)>Pd(NH3)(2)Cl-2>Pd(C2H3O2)(2), which is nearly identical to the catalytic activity. This indicates that the dispersion of palladium plays an important role in the catalytic activity. Furthermore, density of Lewis (L) and Bronsted (B) acid sites are also strongly dependent on the palladium precursors. it is also demonstrated that an effective catalyst should possess a well combination of Bronsted acid sites with dispersion of palladium. (C) 2009 Elsevier B.V. All rights reserved.