Heterogeneous Wacker oxidation catalysts, consisting of a palladium complex supported on a monolayer vanadium pentaoxide on gamma-alumina, have been tested in the oxidation of 1-butene to butanone. The steady-state activity of the catalysts is found to be 0.2 mmol butanone per mol Pd per second. The oxidation of 1-butene over the heterogeneous catalysts runs analogously to the homogeneous Wacker oxidation. The stability of the catalysts depends on the structure of the vanadium pentaoxide overlayer. Continuous reoxidation of Pd-0 is possible only if the vanadium pentaoxide is present in a molecularly dispersed overlayer and does not proceed when crystalline vanadium pentaoxide is used as a support. Temperature-programmed reduction measurements have shown that this is the result of the lower reducibility of the crystalline vanadium pentaoxide as compared to supported vanadium pentaoxide monolayers. The stability of the supported catalysts depends strongly on the vanadium pentaoxide coverage; due to sintering of the vanadium pentaoxide overlayer, a rapid deactivation is observed for catalysts with coverages below 1.5 V/nm(2). Sintering is probably caused by the poor interaction of the partly reduced vanadium pentaoxide layer with gamma-alumina. X-ray photoelectron spectroscopy analysis revealed that under the reaction conditions 35% of the vanadium pentaoxide is present as V4+ and 50% of the palladium is present as Pd-0.