Ionic liquids dissolution/precipitation pretreatment of barley straw was used to improve the yield to produce ethanol. The pretreatment with ionic liquid greatly improves the enzymatic hydrolysis compared to untreated samples, reducing the hydrolysis time to only 24 h, which is lower than untreated samples and samples treated with other methods. The treatment with 1-ethyl-3-methylimidazolium acetate produced a better hydrolysis yield with a glucose yield close to 100%. We produced ethanol with pretreated samples using simultaneous saccharification and fermentation (SSF) and pre-saccharification, saccharification and fermentation (PSSF). In both procedures we obtained very high and similar yields because the pretreatment enhances the enzymatic hydrolysis rate and no pre-saccharification is needed. We obtained 22.9 g ethanol/100 g pretreated barley straw, representing 97% of the potential ethanol from the cellulose, one of the highest yields reported in the literature.