The adsorption and reaction of acetophenone and isotope-labeled acetophenone were investigated on reduced Pd/SiO2, Pd black, and oxygen-dosed Pd/SiO2 via thermal desorption mass spectrometry and FT-IR. The support can strongly influence the decomposition reaction of acetophenone on Pd/SiO2. Acetophenone is initially coordinated to Pd/SiO2 in the eta(1)(O) configuration at room temperature. The eta(1)-acetophenone either desorbs without reaction or converts to eta(2)(O,C)-acetophenone during annealing of the adlayer. The eta(2)-acetophenone starts to decompose and/or desorb at temperatures higher than 50 degrees C. Acetophenone decomposes to benzene, toluene, CO, H-2, and hydrocarbon fragments on Pd catalysts during the course of TPD experiments. Benzene desorbs in alpha, beta, and gamma states, respectively, centered at 135, 180, and 250 degrees C. The alpha state is from the benzene decomposition product of acetophenone adsorbed on the interface between Pd and SiO2. The beta state is from the benzene adsorbed on the Pd metal surface. The gamma state comes partly from the benzene adsorbed on the Pd surface. Some part of the gamma state comes from the decomposition reaction of the acetophenone that migrates from the support to the Pd particle. Toluene desorbs in alpha and gamma states centered at 135 and 250 degrees C. The alpha state toluene comes from the reaction of the beta state benzene and acetyl species at the interface between Pd and SiO2. The gamma state toluene comes from the reaction of the beta state benzene and acetyl species produced from the decomposition reaction of the acetophenone that migrates from the support to the Pd particle.