Thermoelectric power of an yttria stabilized zirconia (YSZ) thermocell, PtYSZPt, has been determined at 1273 K in different atmospheres. The atmospheres used have been mixtures Of O-2-inert gases, CO-CO2, and H-2-H2O. Oxygen partial pressures ranged between 1 and 10(-21) atm. The yttria content in the YSZ was 8 mol%. The dependence on temperature of the thermoelectric power of YSZ in air has also been measured between 600 and 1100-degrees-C. The thermoelectric power was determined to be 0.486 mV/K in air at 1 atm, the colder end was positive relative to the hotter end. The transported entropy of O-2- in yttria-stabilized zirconia is calculated from a combination of thermodynamic and measurement data and is found to be 33 JK-1 mol-1, independent of oxygen partial pressure and temperature. It is shown that the major contribution to the measured thermoelectric power comes from the entropy change of the electrode reactions. It is possible, from the thermoelectric power of a YSZ thermocell, experimentally to determine approximative values of the single electrode reversible heat evolution in an SOFC. These calculations give that 60 kJ/mol e- will be evolved at the cathode when air is the cathode gas and 41 kJ/mol e- will be consumed at the anode in a mixture of 98% H-2 and 2% H2O, at 1273 K when the total gas pressure at both cathode and anode is 1 atm.