The effect of impurities on the degradation of performances was investigated for the flatten tube type SOFC stack. The durability tests of 20-cells stack were conducted at 750 degrees C (1023 K) with dry H-2 for more than 5000 h under a constant current density of 0.3 A cm(-2). The voltage loss showed a linear relationship between voltage loss and operation time (about 1.5%/1000 h). The ohmic resistance increased with operation time while the polarization resistance showed constant values. After the long-term operation test, the concentration levels of impurities were measured at cathode and interlayer by secondary ion mass spectrometry (SIMS). The concentrations of several elements were successfully determined in ppm levels. The concentrations of several elements increased with operation time (Na, Al, Si, and Cr), which suggested the transports and depositions on the cell component surface. The increase of resistance and impurity concentration at the interlayer were estimated from the literature data and SIMS impurity analysis. (C) 2008 Elsevier B.V. All rights reserved.