In times of increased energy consumption and global warming, the development of novel approaches for green energy, is getting more and more important. One promising candidate is the solid oxide fuel cell (SOFC). SOFCs, especially used in the combination with heat exchange devices, are able to achieve high efficiency values and may thus reduce production of green house gases. Nevertheless, there are still some important issues in the area of efficiency and stabilization. Until now, the high number of different electrochemical reactions and materials science related processes are making the investigation and optimization of fuel cells very difficult. One of the most common materials, used for SOFCs, is La0.8Sr0.2MnO3±- (LSM), which enables the oxygen reduction reaction (ORR). It will be further investigated in this work. The LSM was deposited by pulsed laser deposition (PLD) on Y2O3 stabilized ZrO2. Photolithography and subsequent ion beam etching are used to prepare the microelectrodes. Through the introduction of strontium the defect concentration changes, leading to an improved ionic and electronic conductivity. For a better understanding of the reaction kinetics in LSM, electrochemical impedance measurements (EIS) on microelectrodes were performed under different oxygen partial pressure and varying bias voltage. By applying a DC-bias to LSM electrodes, their oxygen chemical potential changes and thus surface kinetics, as well as bulk transport kinetics, can be modified. Especially the bulk path was investigated in detail, using the equivalent circuit developed by Jamnik and Maier, to get a better understanding of the ongoing reaction processes within the material. A user defined fit function, based on the equivalent circuit, allows to extract the necessary data to calculate the ion diffusion coefficient (D q) and the surface exchange coefficient (k q). In addition to these measurements, the influence of the counter electrode (Platin or LSF) on the EIS measurements at low partial pressure was investigated.