In the present work, different crystalline Mn-oxide thin films (MnO, alpha-Mn2O3, and gamma-Mn2O3) were prepared on glass and Si(P) substrates by evaporation Of MnO2 powder followed by annealing in air and in vacuum at different temperatures. The oxide films were characterised by methods of energy-dispersion X-ray fluorescence method (EDXRF), X-ray diffraction (XRD), and ultraviolet-visible (UV-VIS) optical spectroscopy. The samples for electrical study were constructed in the form of metal-oxide-Si (MOS) Structures and characterised by measurements of their capacitance and ac-conductance as a function of gate-voltage. The frequency dependence of ac-electrical properties of Mn-oxide films and their variation with the annealing temperatures or crystal-structural content were studied. The measured values of the relative permittivity for Mn oxides of different crystal structures were tabulated. It was found that the obtained MnO oxide because of the annealing in vacuum at about 500 degrees C has the highest relative permittivity, about 10, Which suggests it to be a candidate for high-k insulator on Si applications. It was found that the data of the ac-measurements follow the correlated barrier-hopping (CBH) model and the model's parameters were determined. (c) 2005 Elsevier B.V. All rights reserved.