The chemical composition of brewer's spent grain (BSG), generated from a process using 100% malted barley, was investigated. BSG is mainly composed of (g kg−1) hemicellulose (284), lignin (278), cellulose (168) and protein (152.5), but it also contains extractives and ashes in smaller proportions. Minerals in BSG include calcium, sodium, potassium, magnesium, aluminum, iron, barium, strontium, manganese, copper, zinc, phosphorus, sulfur, chromium and silicon. In order to find alternative uses for BSG, this by-product was subjected to acid hydrolysis process to recover the hemicellulosic sugars, which have a large number of industrial applications. The process was carried out under different conditions of liquid-to-solid ratio and acid concentration to evaluate the influence of these two variables on BSG hemicellulose hydrolysis. Under all the hydrolysis conditions evaluated, arabinose was recovered with higher efficiency than xylose. Under the best evaluated reaction conditions (liquid-to-solid ratio of 10 g g−1 and 120 mg H2SO4 g−1 dry matter) 76.2% of the hemicellulose was hydrolyzed and the xylose and arabinose sugars were recovered with 67 and 97.8% efficiency, respectively.