One of the key issues of resistive switching memory devices is the so called "forming" process, a one time process at a high voltage, which initializes the resistive switching at significantly lower voltages. With this study we identify the influence of the different layers - namely the insulating oxide layer (ZrO2 and Ta2O5) and the reactive ohmic electrode layer (Hf, Ta and Pt) - on the forming voltage and the pristine capacitance of the devices. For this, the forming voltage and pristine capacitance is measured in dependence of the oxide layer thickness with different electrodes. The different slopes of the forming voltage - thickness relation for different top electrodes give an indication that the reactive ohmic electrode is oxidized from the oxide layer underneath and that the degree of the oxidation depends on the thickness of the oxide layer as well as the materials used for the oxide and electrode layer. This finding could be confirmed by X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) measurements. From the electrical measurements and the TEM images the thickness of the oxidized electrode layer could be estimated. The degree of the oxidation depends on the oxygen affinity of the oxide and electrode material. The interface dependent (thickness independent) part of the forming voltage is determined by the material of the electrode. The magnitude of this interface voltage could be correlated to the oxide free energy of the electrode material. These results can support the ongoing research towards resistive switching memory devices with a very low forming voltage or forming free behaviour.