Display omitted ▶ 3D tomography showed the high temperature evolution of Ni-YSZ anodes. ▶ Ni particle size changed little, constrained by the high vol% YSZ matrix. ▶ Pore percolation decreased and interfacial area decreased. ▶ The pore evolution substantially decreased the electrochemically active TPB. ▶ The loss of active TPBs explained observed anode polarization resistance increase. Temperature induced degradation in Solid Oxide Fuel Cell (SOFC) Ni-YSZ anodes was studied using both impedance spectroscopy and three-dimensional tomography via Focused Ion Beam–Scanning Electron Microscopy. A 100h anneal at 1100°C caused a 90% increase in cell polarization resistance, which correlated with the observed factor of ∼2 reduction in the electrochemically active three-phase boundary (TPB) density. The TPB decrease was caused by a significant decrease in pore percolation, and a reduction in pore interfacial area due to pores becoming larger and more equiaxed. The anneal caused no measurable change in average Ni particle size; Ni coarsening was apparently highly constrained in these anodes due to the relatively large YSZ volume fraction and low pore volume.