Acetogens have often been observed to be inhibited by CO above an inhibition threshold concentration. In this study, a two-stage culture consisting of carboxydotrophic archaea and homoacetogenic bacteria is found to be effective in converting industrial waste gas derived from a steel mill process. In the first stage, Thermococcus onnurineus could grow on the Linz-Donawitz converter gas (LDG) containing ca. 56% CO as a sole energy source, converting the CO into H-2 and CO2. Then, in the second stage, Thermoanaerobacter kivui could grow on the off-gas from the first stage culture, consuming the H-2 and CO in the off-gas completely and producing acetate as a main product. T. kivui alone could not grow on the LDG gas. This work represents the first demonstration of acetate production using steel mill waste gas by a two-stage culture of carboxydotrophic hydrogenogenic microbes and homoacetogenic bacteria. (C) 2016 Published by Elsevier Ltd.