To utilize waste CO for mixed culture gas fermentation, carbon sources (CO, CO2) and pH were optimized in the batch system to find out the center point and boundary of response surface method (RSM) for higher acetate (HAc) production (center points: 25% CO, 40% CO2, and pH 8). The concentrations of CO and CO2, and pH had significant effects on acetate production, but the pH was the most significant on the HAc production. The optimum condition for HAc production in the gas fermentation was 20.81% CO, 41.38% CO2, 37.81% N-2, and pH 7.18. The continuous gas fermentation under the optimum condition obtained 1.66 g/L of cell DW, 23.6 g/L HAc, 3.11 g/L propionate, and 3.42 g/L ethanol. (C) 2016 Elsevier Ltd. All rights reserved.