Palladium salts of heteropolyacids (PdHPAs) of the Keggin series Ha H3+nPVnMo12-nO40 supported on silica, have been used successfully as catalysts in the gas-phase Wacker oxidation of 1-butene. In such catalysts the palladium reaction centre and the redox component are combined in one complex. At 343 K and atmospheric pressure a high initial butanone yield of more than 0.2 g g(cat)(-1) h(-1), in combination with a very high butanone selectivity of more than 98%, can be obtained. In the steady state, the activity of the catalyst is more than a factor of 10 lower than the initial activity, due to slow reoxidation of reduced palladium-heteropolyanion complexes. The rate of reoxidation depends on the composition of the HPA, the palladium loading, and the reaction conditions. The reaction order of 0.5 in the O-2 partial pressure indicates the dissociation of dioxygen to be rate determining. The degree of hydration of the HPA appears to be important for the activity and stability of the catalysts. Spent catalysts can be regenerated by an oxidation treatment in air at temperatures around 525 K. Regeneration becomes more difficult with high palladium loading of the catalyst.