The behavior of carbon- and silica-supported Pd catalysts made from Pd acetylacetonate was compared to each other and also to carbon-supported Pd prepared from PdCl2. Suppressed chemisorption and hydride formation was observed after standard hydrogen reduction pretreatments at 573 K which was attributed to carbon contamination of the Pd bulk and surface. Different treatments with oxygen in addition to pretreatment sequences with and without hydrogen reduction followed by hydrogen, oxygen and carbon monoxide chemisorption or hydrogen and oxygen titration measurements were used to clarify the source of the carbon contaminant. Initial contamination due to the Pd(AcAc)(2) precursor was observed but it could be removed by calcination at 573 K in 4% oxygen; however, subsequent recontamination during reduction in hydrogen indicated that carbon atoms from the support could also be involved. Protecting the Pd surface with chemisorbed oxygen by cooling in oxygen prevented this carbon contamination.