Hydrolysates from sweet sorghum bagasse pretreatment normally contains hexose and pentose sugars, and this complex mixture of sugars presents a challenge for a single microorganism to effectively ferment all sugars to ethanol. In this study, synergistic effects on the co-fermentation of the hydrolysates using Sacchromyces cerevisiae and Zymomonas mobilis ATCC31825 at different ratios were studied. An inoculum of mixed cultures (1:3 and 5: 10 g/L, Z. mobilis to S. cerevisiae ratios) was investigated. Each mixed culture was added to the hydrolysates at pH 4.8 and incubated 32 degrees C for 24 h. The mixture of Z. mobilis to S. cerevisiae at 5: 10 g/L showed the highest synergistic effect with ethanol yields of 0.5 g/g. Since the yield for co-culture was significantly higher than the sum of yields from each microorganism, the improvements can be directly related to co-fermentation of hydrolysate by S. cerevisiae and Z. mobilis. (C) 2014 Elsevier Ltd. All rights reserved.