The thermodynamic data for the adsorption of pyridine, N-methylimidazole, and dimethylcyanamide from cyclohexane onto palladium oxide crystallites supported on carbon (5% w/w) were obtained by calorimetric and adsorption studies. The data indicated that there are at least two acceptor sites on the palladium oxide surface. Both the calorimetric and adsorption experiments give the same saturation adsorption capacity for site 1. Its magnitude suggests that the adsorbed pyridine is normal to the surface plane of palladium oxide, indicating that the adsorption process involves a Lewis acid-base type of interaction. In the low-coverage region, the adsorption quotients of base onto palladium oxide are considerably larger than those of carbon, indicating a preferential adsorption of base onto the palladium oxide surface.