Microstructural changes in solid oxide fuel cell anodes after long-term operation have been characterized by sequential sectioning with a focused ion beam, followed by scanning electron microscopy imaging and three-dimensional reconstruction. The anodes were porous composites of Ni and Y2O3-stabilized ZrO2 (YSZ). The cells were operated at 800 degrees C for 2, 4, and 8 kh, and at 925 degrees C for 2 and 4 kh. For each specimen, the volume fraction, surface area, particle diameter, and tortuosity have been calculated for each phase (Ni, YSZ, and pores). The dependence of these microstructural parameters on the volume of sample analyzed was monitored; sufficiently large volumes were analyzed so as to eliminate any effect of sample volume. Gradients in volume fraction of Ni and porosity developed during fuel cell operation, with Ni fraction increasing, and pore fraction decreasing, at the electrolyte/anode interface. The magnitudes of these gradients increased with time.