Performance degradation data obtained on single solid oxide fuel cells tested at 850°C with air and humidified H2 and using Ni-YSZ anode supported cells are presented. Microscopic investigation is carried out on both anode and cathode to quantify variations in morphology at different operation times. The comparison between the measurements on the cells and the SEM image analysis allows concluding that there is no relationship between the initial cell activation and micro-structural modifications of the electrodes. On the other hand, it was found that cell degradation is strictly related to the coarsening of Ni particles occurring in the anode. A theoretical analysis based on an electrode micro-model has been performed in order to compare the variation in performance, expected from particle size change, with the observed data. The model confirmed the conclusions taken from the experimental results.