Time dependent properties and performance of tubular solid oxide fuel cells were studied numerically and experimentally. The numerical model incorporated local characteristics such as porosity, tortuosity, grain size, and conductivity and was used to evaluate the specific and relative changes in performance caused by the effect of time-dependent material changes of those characteristics. A 500 hour experimental study was conducted at in on an extruded electrolyte-supported tubular SOFC made in our laboratory. Changes in current density with time (at constant voltage) formed a curve with initial convex (upward) curvature, becoming monotonic decreasing. The microstructure of the constituent layers was examined by scanning electron microscopy. Comparisons between model predictions and experimental observations were made. For the situation modeled and tested, the porosity and ionic conductivity were found to be most influential on performance. More importantly, the effect of porosity is a trade-off between the influence on gas transport and the mixed conductor influence on the electrochemical reactions at the electrode.