This study explored CO as a primary fuel in anode-supported solid oxide fuel cells (SOFCs) of both tubular and planar geometries. Tubular single cells with active areas of 24 cm(2) generated power up to 16W. Open circuit voltages for various CO/CO2 mixture compositions agreed well with the expected values. In flowing dry CO, power densities up to 0.67 W cm(-2) were achieved at 1 A cm(-2) and 850 degrees C. This performance compared well with 0.74 W cm(-2) measured for pure H-2 in the same cell and under the same operating conditions. Performance stability of tubular cells was investigated by long-term testing in flowing CO during which no carbon deposition was observed. At a constant current of 9.96 A (or, 0.414 A cm(-2)) power output remained unchanged over 375 h of continuous operation at 850 degrees C. In addition, a 50-cell planar SOFC stack was operated at 800 degrees C on 95% CO (balance CO2), which generated 1176W of total power at a power density of 224 mW cm(-2). The results demonstrate that CO is a viable primary fuel for SOFCs. (C) 2010 Elsevier B.V. All rights reserved.