Microtubular solid oxide fuel cells with La0.6Sr0.4Co0.2Fe0.8O3.8 (LSCF) cathodes were aged in ambient air at temperatures ranging from 700 to 1000 degrees C for up to 700 h. Distribution of relaxation time analysis of electrochemical impedance spectroscopy (EIS) measurements indicated an increase in the cathode resistance with increasing ageing temperature up to 900 degrees C. The cells aged at 1000 degrees C were not measurable. The degradation did not result from microstructural changes, as shown by focused ion beam-scanning electron microscopy (FIB-SEM) 3D tomography that showed no significant changes at ageing temperatures <= 900 degrees C. On the other hand, the amount of segregated Sr, present both on LSCF surfaces and in Sr-rich particles, increased with increasing ageing temperature and correlated well with the increase in cathode resistance. Analysis of the EIS and microstructural data using the Alder-Lane-Steele model indicates that the Sr surface segregation increases cathode resistance via a decreased oxygen surface exchange rate.