A solid oxide fuel (SOFC) cogenerates heat and electricity with high efficiency. Electricity is produced by the electrochemical reaction of a fuel (H2, CO, CH4) and an oxidant gas (O2, air). The SOFC standard design is based on an anode-supported electrolyte. The anode is composed of yttria-stabilized zirconia (YSZ) and nickel oxide (NiO). NiO is reduced in situ into metallic nickel during the first use of the fuel cell. Porosity is formed to compensate the volume shrinkage induced by the reduction (40 vol%). The nickel catalyst may reoxidize due to different factors. Nickel volume change (70 vol%) is not accommodated completely by the internal porosity. Stress is produced in the YSZ backbone. The performances of the SOFC degrade due to the creation of electrolyte cracks. The reason behind the irreversible effect of a redox cycle on Ni-based anodes is subject to controversies. Research groups suggest that the expansion of Ni upon a redox cycle is either caused by the reorganization of nickel during reduction or by formation of porosity upon reoxidation. Time resolved environmental transmission electron microscopy (ETEM) was used here to characterize in situ the reduction and reoxidation mechanisms of Ni-based anodes and thus understand the exact causes of this degradation mechanism.