The vast majority of catalysts used in modern chemical industry are oxides. Because of their ability to take part in the exchange of electrons, as well as in the exchange of protons or oxide ions, oxides are used as catalysts in both redox and acid-base reactions. They constitute the active phase not only in oxide catalysts but also in the case of many metal catalysts, which in the conditions of catalytic reaction are covered by a surface layer of a reactive oxide. Properties of oxides are also important in the case of preparation of many metal and sulfide catalysts, which are obtained from an oxide precursor. There are many different experimental evaluation techniques to study oxides, that offer interesting information, but all of them have several limitations.  Although the primary application of EIS was investigation of the kinetics of electrode reactions, currently it can be successfully applied to determine properties of materials. Electrochemical impedance spectra often are interpreted based on the fitting results to equivalent electric circuit (EEC), and elements of EEC have clear physical meanings. In many cases, the EEC shown in Fig. 1 might be applied to explain material electrochemical behavior in aqueous solutions. R1=Rs, which is uncompensated resistance, dependently on the investigating system C1 is either capacitance of double electric layer or capacitance of porous layer, R2 is either charge transfer resistance either is porous layer resistance, the circuit containing C2 and R3 describes either kinetics of sequent reactions either C2 and R3 is capacitance and resistance of barrier layer, respectively. For example, based on this EEC it is possible to prove formation of oxide monolayers during corrosion of Co-W alloys. Another example is a creating of Mott-Schottky plot based on determined double layer capacitance and further investigation properties of semiconductor oxide layers (see Fig.2). Frequently, capacitors are exchanged by constant phase element due to inhomogeneity of local impedances. Fig. 1. Equivalent circuit model employed in analysis of electrochemical impedance data of complicated electrode processes. Fig. 2. An example of Mott-Schottky plot for passive film.