Inhibitory compounds generated by the pretreament of lignocellulose processes can have negative impacts on downstream microbial growth and biofuel production. As such they present a significant barrier to the commercialization of biofuels produced using renewable carbon sources. Inhibitors are derived from the transformation of monosaccharides into toxic compounds and can also be an intrinsic component of the pretreatment reagents or biomass. Ionic liquid (IL) pretreatment has recently received attention as a potential alternative to established pretreatment techniques, but there are no published studies on its impact on downstream processes. In this study, sugars produced through the enzymatic hydrolysis of biomass pretreated with the IL 1-ethyl-3-methylimidazolium acetate ([C₂mim][OAc]) were used to assess the impact of IL pretreatment on cell growth and production of ethanol by Saccharomyces cerevisiae. Residual 1-ethyl-3-methylimidazolium ion ([C₂mim]⁺) in hydrolysates was found to be the primary source of inhibition on downstream microbial growth and ethanol production.