The complex admittance of the Si+/SiO2/Pentacene/Au (metal/oxide/pentacene) thin film junctions is investigated under ambient conditions. The results are compared with the ones obtained for the corresponding Si+/SiO2/Au junctions (i.e. a small part of the surface left free from pentacene) which constitutes the “reference” of our samples. This allows us to extract the “organic” part of the dielectric response from the whole spectrum. Our data clearly show that the admittance is decomposed in three main contributions. At low frequencies, a contribution attributed to proton diffusion through the oxide is seen. This diffusion is shown to be anomalous and is believed to be also at the origin of the bias stress effect observed in organic field effect transistors. At higher frequencies, two dipolar contributions are evidenced, attributed to defects located one at the organic/oxide interface or within the organic, and the other in the bulk of the oxide. These two dipolar responses show different dynamic properties that manifest themselves in the admittance in the form of a Debye contribution for the defects located in the oxide, and of a Cole-Cole contribution for the defects related to the organic.