Lignocellulosic biomass, primarily comprised of cellulose, hemicellulose, and lignin, is a promising renewable feedstock for the microbial production of chemicals, especially ethanol.  The major fermentable sugars (hydrolysates) released from the processing of the lignocellulose are glucose and xylose.  However, the primary processing steps required for this conversion also produce a range of compounds that can inhibit the subsequent microbial fermentation.  One such inhibitory compound is acetic acid, liberated during the pretreatment of the biomass.  In this poster, we report the effect of acetic acid on glucose/xylose co-fermentation by the genetically modified S. cerevisiae 424A(LNH-ST).  The co-fermentation of glucose and xylose was performed under acetic acid conditions of 5, 10, 15 g/L over a pH range of 5 – 6.   To maintain the pH at the specified initial value, the fermentations were carried out in a 1L New Brunswick BioFlow 110 benchtop fermentor equipped with a pH controller.  Results showed that the fermentation of both sugars was affected by the presence of acetic acid.  The inhibitory effect of acetic acid increased as the pH decreased.  The results also indicate that the utilization of xylose is more influenced by acetic acid concentration and pH than the utilization of glucose.