The role of Te in Pd-Te-H4SiW12O40/SiO2, a highly active and selective catalyst for direct oxidation of ethylene to acetic acid, was investigated. Kinetic studies of ethylene oxidation over Pd-H4SiW12O40/SiO2 and Te-promoted Pd-H4SiW12O40/SiO2 demonstrated that doping with Te enhanced the rate of reaction of ethylene to acetaldehyde but reduced direct formation of CO2 from ethylene. In contrast, the successive reaction of acetaldehyde to acetic acid was not influenced to a significant extent by the addition of Te.