Heteropoly acid (HPA) salts containing transition metal cations have been applied as reoxidants of palladium(II) in heterogeneous Wacker catalyst for the oxidation of ethene to acetaldehyde. It has been found that the presence of transition metal cations increases the initial ethene conversion and influences the heteropoly anion reducibility. The steady state activity has been shown to be inversely proportional to the facility of oxygen removal from the HPA salts.