The transition metal salts of heteropoly acids have been applied as palladium re-oxidants in heterogeneous solid Wacker catalysts for the oxidation of butene-l to MEK. Silica, gamma-alumina and titania were used as supports for the Pd/MeHPMoVx, system. The influence of cations bonded to heteropoly anions and also the effect of the catalysts acidity on the butene-l to MEK oxidation have been elucidated.