An anode support planar SOFC with a 4 x 4-cm(2) active area was fabricated with LSM (La(0.78)nSr(0.22)Mn(x)O(3)), 8YSZ and Ni/YSZ. A single cell was tested with hydrogen and methane fuels to compare cell performance and to investigate carbon deposition effects. Catalytic activity of anode support was characterized by AC impedance analysis, and cell performance was measured with current-voltage curves. Impedance responses for the anode support cell showed one large arc at high frequency, which is considered a typical indication of polarization resistance from the anode support. A carbon deposition test was conducted with flowing methane at an open-circuit condition for a significant period. We observed an increase of arc from the comparison of impedance responses before and after the carbon deposition. The increased impedance arc recovered to its original magnitude when electrical load applied to the cell so that sufficient oxygen ions are supplied. This reversibility of carbon deposition resulted in no change of cell performance. The impedance change we observed for carbon deposition was clearly distinguished from what we consider oxidation of Ni that was observed from impedance analysis for the cell exposed to a mildly oxidative condition. Irreversible carbon deposition was also tested by running the cell at a thermodynamically favorable condition for carbon deposition, and a drop of cell performance was observed.