The gas-phase selective hydrogenation of phenol to cyclohexanone has been investigated over supported Pd catalysts in order to clarify the role of the support, the Pd precursor and the addition of Ca in controlling activity, selectivity and stability of the catalytic system. The catalytic results showed that over monometallic Pd catalysts prepared by PdCl2 as precursor the order of activity and selectivity to cyclohexanone was the following: Pd/La2O3 > Pd/CeO2 > Pd/Al2O3. The same order of activity was observed over ex-Pd(CH3COO)(2) catalysts. However, over these latter samples the selectivity to cyclohexanone was very high regardless of the support used. Addition of calcium strongly improved the catalytic performance of both ex-chloride and ex-acetate Pd catalysts supported on alumina, whereas no significant improvement was observed on Pd catalysts supported on CeO2 and La2O3. On the basis of characterization data (CO chemisorption, TPD and FT-IR) reported in this paper it has been suggested that the acid-base properties of supported Pd catalysts strongly influence the adsorption-desorption equilibrium of the reactant and products, being responsible for directing the selectivity to the reaction products. The catalytic activity of the system appears to be, instead, mainly controlled by the palladium sites on which hydrogen is activated.