The high-temperature corrosion behavior of Co-Al binary alloys (containing Co-2, 5, and 10wt.% Al) was studied over the temperature range 600-800℃ in H2/H2S/H2O gas mixture. Generally, the corrosion kinetics of all the alloys followed the parabolic rate law, and the corrosion rate decreased with increasing Al content. The kinetics inversion was observed between 700 and 800℃ for higher Al alloys (>5%Al). The scales formed on the alloys were duplex, consisting of an outer-layer of Co9S8 and of an inner-layer of mostly Al2S2, AL2O3, and minor amounts of Co9S8.