Abstract. Solid oxide fuel cells directly convert the chemical energy of a fuel into electricity. To enhance the efficiency of the fuel cells, the thickness of the gastight solid electrolyte membranes should be as thin as possible.Y203-stabilised ZrO 2 (YSZ) electrolyte films were prepared by reactive sputtering deposition using Zr/Y targets in Ar/O 2 atmospheres. The films were 5 -8 p.m thin and were deposited onto anode substrates made of a NiO/YSZ composite. After deposition of a cathode with the composition Lao.65Sr0 ~5MnO3 the electrochemical properties of such a fuel cell were tested under operating conditions at temperatures between 600 ~ and 850 ~ Current-voltage curves were recorded and impedance measurements were performed to calculate apparent activation energies from the fitted resistance data. The conductivity of the YSZ films varied between 4.6.10-6S/cm and 2.2.10 .5 S/cm at 400 ~ and the fuel cell gave a reasonable power density of 0.4 W/cm 2 at 0.7 V and 790 ~ using H~ with 3 % H=O as fuel gas. The gas compositions were varied to distinguish the electrochemical processes of the anode and cathode in the impedance spectra.