Solid Oxide Fuel Cells (SOFCs) are gaining more and more importance as auxiliary power units (APU), e.g. for passenger cars, trucks and airplanes. In this context, the main challenge is the development of SOFC stacks, which fulfill the strong requirements for mobile applications. These are a low weight, a low volume, and a high power density with reformate gases but also low long term degradation rates. The paper presents results of investigations of SOFC short stacks for mobile applications. Therefore, a light weight stack design was developed in an industrial consortium in cooperation with the German Aerospace Center (DLR) in Stuttgart and the Research Center Jülich (FZJ). The SOFC stacks were operated at different temperatures, varying fuel gas compositions and different fuel gas flow rates. The short stacks were electrochemically characterized mainly by long-term measurements, by current-voltage measurements and by impedance spectroscopy. The fuel utilizations and the power densities were determined. In order to analyze the uniformity inside the stacks, the electrochemical behaviour of the individual cassettes were analyzed and compared to each other. Finally, the degradation rates and the degradation mechanisms were determined from the long-term measurements.