The corrosion behavior of a Co-Ce alloy containing approximately 15wt% Ce has been studied at 600-800 o C in several H 2 -H 2 S-CO 2 mixtures, providing sulfur pressures of 10 - 8 atm at 600, 700 and 800 o C and of 10 - 7 atm at 800 o C, and oxygen pressures of 10 - 2 4 atm at 600 o C and 10 - 2 0 atm at 700 and 800 o C. At 600 o C, the alloy corrodes more slowly than pure cobalt but more rapidly than pure cerium while, at 700 o C, it corrodes at about the same rate as pure cerium, but much faster than pure cobalt. At 800 o C under 10 - 8 atm S 2 , i.e. a value below the stability of the cobalt sulfides, the alloy corrodes rather slowly but, under 10 - 7 atm S 2 , the rate is very high, although slightly lower than that of pure cobalt. The scaling kinetics are generally intermediate between linear and parabolic but are sometimes irregular. The corrosion of this alloy produces multilayered scales, containing an outermost layer of almost pure cobalt sulfide, an intermediate complex layer composed of a mixture of compounds of the two metals and, finally, an innermost region of internal attack of cerium by both oxygen and sulfur. Cerium is not able to diffuse outwards and remains in the alloy consumption region. In the intermediate region cobalt sulfide forms a continuous network which allows the growth of the external CoS y layer, although at rates that are reduced with respect to those of pure cobalt. Thus, a cerium content of 15wt% is not sufficient to prevent or even to significantly reduce the sulfidation of the base metal. These results, as well as the details of the microstructure of the scales, are interpreted by taking into account the limited solubility of cerium in the base metal and the presence of an intermetallic compound, rich in cerium, in the alloy.