Thin Pd oxide films formed by holding at potentials up to 1.8 V vs. RHE and in single sweeps up to 2.2 V at polycrystalline Pd electrodes in both acidic and basic solutions were investigated with the use of ac impedance techniques. At both acidic and alkaline pH, impedance analysis shows that, once about one monolayer of the compact alpha-oxide film is formed, it exhibits dielectric properties which do not change over the potential and film thickness range investigated. A permittivity of 19 obtained for oxide grown in 1 mol 1-1 sulfuric acid and 40 when formed in 1 mol 1-1 NaOH, suggesting that the alpha-oxide film has different properties in these two different media. In alkaline solutions, evidence is obtained for the potential-dependent adsorption of hydroxide ion on Pd at potentials prior to alpha-oxide formation. The presence of a pseudo-capacitance in the case of oxide-coated electrodes in this medium also suggests hydroxide adsorption or the presence of a hydrated layer on the alpha-Pd oxide film surface. At potentials greater than ca. 2.0 V, the ac response changes significantly and can be correlated with the onset of growth of a second type of Pd oxide, considered to be the hydrous beta-oxide film material.