Pretreatment of lignocellulosic biomass, while improving enzymatic digestibility, can also produce fermentation inhibitors such as furfural, HMF, and acetic acid. These compounds can decrease the fermentability and the ethanol yields from sugars derived from lignocellulose. This paper reports a systematic study of the effect of furfural, HMF, and acetic acid on the fermentation of both glucose and xylose to ethanol by the recombinant yeast S. cerevisiae 424A(LNH-ST). Batch fermentations (30°C, 200 rpm) were conducted with inhibitors in a control solution of YEP with glucose and xylose as co-substrates. Inhibitor concentrations were varied from 0 to 40 g/L. The results show that concentrations of these inhibitors below approximately 5 g/L cause negligible inhibition for yeast cells in early stationary phase. We confirm that furfural is more inhibitory than HMF and that the toxicity of acetic acid is pH dependent. A preliminary model of glucose/xylose co-fermentation by S. cerevisiae 424A(LNH-ST) with the effect of these inhibitors is presented.