Xylose consumption by Saccharomyces cerevisiae 424A(LNH-ST) during simultaneous saccharification and co-fermentation (SSCF) of AFEX(TM) pretreated switchgrass was inhibited by unhydrolyzed solids. Such inhibitory effects were not found in unhydrolyzed solids from AFEX(TM) pretreated corn stover (AFEX(TM)-CS). However, the xylose consumption was still unsatisfactory during 6h pre-hydrolysis SSCF. By extending the pre-hydrolysis time to 24h or longer, the xylose consumption was improved significantly. In order to better understand the reasons for such improvement, the hydrolysate slurries after 6h pre-hydrolysis and 24h pre-hydrolysis were studied and compared. We found that the glucose concentration after pre-hydrolysis was the critical factor that determined cell viability and hence xylose consumption during SSCF. Low temperature (30°C) and ethanol inhibition were shown to be the factors limiting hydrolysis rate and hence productivity during SSCF.