This paper deals with the characterization of brewer's spent grain (BSG) and the optimization of the phosphoric acid pretreatment for this feedstock. The influence of temperature and acid concentration on BSG was studied, and the optimal conditions were found to be 155 degrees C and 2% H3PO4. The use of both pretreatment and enzymatic hydrolysis together recovered 92% of total sugars in BSG, mainly solubilized in the prehydrolysate (63%). Escherichia coli SL100 fermented this mixed sugar solution containing hemicellulosic sugars and starchy glucose without previous detoxification with an ethanol yield of 0.40 g/g. Considering also the glucose released from the cellulosic structure and converted to ethanol by a simultaneous saccharification and fermentation process, an overall ethanol yield of 17.9 g of ethanol per 100 g of raw BSG was achieved. Thereby, the process configuration proposed in this work allowed 69% of the total sugars in the BSG to be converted to ethanol.